[Rust] Do not escape bit set choice names used in accessors

[s1amese2003]

Fixes #1118.

What

A set choice named after a Rust keyword generates accessors that do not parse:

pub fn get_r#type(&self) -> bool { ... }
pub fn set_r#type(&mut self, value: bool) -> &mut Self { ... }

error: prefix `get_r` is unknown
  --> src/flags.rs:16:12
   |
16 |     pub fn get_r#type(&self) -> bool {
   |            ^^^^^ unknown prefix

The generated Debug implementation calls the same names, and prints the
escaping in its labels.

Why

generateSingleBitSet formats the choice name with formatFunctionName, which
escapes a name that shadows a Rust keyword as a raw identifier. A choice is only
ever reached through a get_ or set_ prefixed accessor, so the prefixed name
is already a plain identifier and cannot shadow a keyword. Escaping the choice
name is therefore never needed, and it places the r# in the middle of the
accessor, where it does not parse.

For the same schema C++ generates type() and Go generates Type..., so Rust
was the only language mangling these names.

Checks

• ./gradlew runRustTests — 36 passed, 0 failed
• ./gradlew :sbe-tool:test — pass
• ./gradlew :sbe-tool:checkstyleMain :sbe-tool:checkstyleTest — pass

Regenerating every Rust codec before and after the change leaves all 137
generated files byte-identical except issue1118/src/flags.rs. The
non-keyword choice in the new schema, and the existing bit sets such as
OptionalExtras, are unchanged.

Reverting only the generator change makes runRustTests fail to compile, so the
new schema keeps this covered.