Optimize String.chunk with uniform fast-path and sub-binary slicing

[preciz]

Assisted-by: Antigravity CLI:Gemini 3.8 Flash

Add fast-paths for 100% valid and printable strings to return [string]
immediately, avoiding the chunk loop in those cases.

In the fallback chunk loop:

• Reuse the first decoded codepoint and classification, eliminating
  redundant decoding and the intermediate wrapper.
• Replace repeated <> binary concatenation with zero-copy sub-binary
  slicing via binary_part/3, removing intermediate binary reallocations.

Bench:

# Run with: elixir bench.exs
Mix.install([:benchee])

path = "lib/elixir/lib/string.ex"

for {ref, module} <- [{"HEAD^1", Old}, {"HEAD", New}] do
  {source, 0} = System.cmd("git", ["show", "#{ref}:#{path}"], cd: __DIR__)

  source
  |> String.replace("defmodule String do", "defmodule #{inspect(module)} do")
  |> Code.compile_string(path)
end

Benchee.run(
  %{
    "HEAD^1" => fn {string, trait} -> Old.chunk(string, trait) end,
    "HEAD" => fn {string, trait} -> New.chunk(string, trait) end
  },
  inputs: %{
    "11B valid" => {"hello world", :valid},
    "11B printable" => {"hello world", :printable},
    "1200B valid" =>
      {String.duplicate("The quick brown fox jumps over the lazy dog! ✨ ", 25), :valid},
    "1200B printable" =>
      {String.duplicate("The quick brown fox jumps over the lazy dog! ✨ ", 25), :printable},
    "1200B mixed valid/invalid" =>
      {String.duplicate("hello" <> <<255, 254>> <> "world" <> <<128>>, 80), :valid},
    "1200B mixed printable" =>
      {String.duplicate("abc\x00def\tghi\x1b[0m", 80), :printable}
  },
  pre_check: :all_same,
  warmup: 1,
  time: 3,
  memory_time: 1
)

Results:

Operating System: Linux
CPU Information: AMD Ryzen 7 8845HS w
Number of Available Cores: 16
Available memory: 54.72 GB
Elixir 1.20.4
Erlang 29.0.5
JIT enabled: true

Benchmark suite executing with the following configuration:
warmup: 1 s
time: 3 s
memory time: 1 s
reduction time: 0 ns
parallel: 1
inputs: 11B printable, 11B valid, 1200B mixed printable, 1200B mixed valid/invalid, 1200B printable, 1200B valid
Estimated total run time: 1 min
Excluding outliers: false

##### With input 11B printable #####
Name             ips        average  deviation         median         99th %
HEAD         13.42 M       74.50 ns   ±474.77%          70 ns         170 ns
HEAD^1        1.17 M      856.65 ns   ±721.97%         771 ns        1543 ns

Comparison: 
HEAD         13.42 M
HEAD^1        1.17 M - 11.50x slower +782.14 ns

Memory usage statistics:

Name      Memory usage
HEAD         0.0547 KB
HEAD^1         2.30 KB - 42.14x memory usage +2.25 KB

**All measurements for memory usage were the same**

##### With input 11B valid #####
Name             ips        average  deviation         median         99th %
HEAD         16.83 M       59.41 ns  ±2402.20%          50 ns          90 ns
HEAD^1        1.34 M      748.97 ns   ±901.97%         641 ns        1522 ns

Comparison: 
HEAD         16.83 M
HEAD^1        1.34 M - 12.61x slower +689.56 ns

Memory usage statistics:

Name      Memory usage
HEAD         0.0547 KB
HEAD^1         2.30 KB - 42.14x memory usage +2.25 KB

**All measurements for memory usage were the same**

##### With input 1200B mixed printable #####
Name             ips        average  deviation         median         99th %
HEAD         22.21 K       45.02 μs    ±31.57%       43.91 μs       52.02 μs
HEAD^1       11.65 K       85.80 μs    ±35.88%       80.39 μs      144.72 μs

Comparison: 
HEAD         22.21 K
HEAD^1       11.65 K - 1.91x slower +40.78 μs

Memory usage statistics:

Name      Memory usage
HEAD         202.66 KB
HEAD^1       245.34 KB - 1.21x memory usage +42.69 KB

**All measurements for memory usage were the same**

##### With input 1200B mixed valid/invalid #####
Name             ips        average  deviation         median         99th %
HEAD         24.51 K       40.80 μs    ±35.33%       39.56 μs       50.85 μs
HEAD^1       14.48 K       69.06 μs    ±43.98%       59.84 μs      167.18 μs

Comparison: 
HEAD         24.51 K
HEAD^1       14.48 K - 1.69x slower +28.26 μs

Memory usage statistics:

Name      Memory usage
HEAD         181.38 KB
HEAD^1       215.33 KB - 1.19x memory usage +33.95 KB

**All measurements for memory usage were the same**

##### With input 1200B printable #####
Name             ips        average  deviation         median         99th %
HEAD        228.93 K        4.37 μs    ±45.77%        4.28 μs        5.56 μs
HEAD^1       13.90 K       71.95 μs    ±32.30%       70.42 μs       79.90 μs

Comparison: 
HEAD        228.93 K
HEAD^1       13.90 K - 16.47x slower +67.58 μs

Memory usage statistics:

Name      Memory usage
HEAD         0.0547 KB
HEAD^1       239.61 KB - 4381.43x memory usage +239.55 KB

**All measurements for memory usage were the same**

##### With input 1200B valid #####
Name             ips        average  deviation         median         99th %
HEAD        694.59 K        1.44 μs    ±63.08%        1.37 μs        1.73 μs
HEAD^1       19.23 K       52.00 μs    ±35.20%       50.45 μs       60.07 μs

Comparison: 
HEAD        694.59 K
HEAD^1       19.23 K - 36.12x slower +50.56 μs

Memory usage statistics:

Name      Memory usage
HEAD         0.0547 KB
HEAD^1       239.61 KB - 4381.43x memory usage +239.55 KB

**All measurements for memory usage were the same**

[preciz]

Needs more work, closing it for now.