Design for StrCtx that takes delimiter or maximum size

[m4b]

See initial discussion in #108 (comment)

We may want to change behavior of DelimiterUntil, but probably more likely we'd add a ctx with this behavior.

Here are the only uses of DelimiterUntil in goblin:

src/pe/symbol.rs
207:                .pread_with(offset, ctx::StrCtx::DelimiterUntil(0, 8))
466:            .pread_with(offset, ctx::StrCtx::DelimiterUntil(0, length))

Either way it's a breaking change since we don't have non-exhaustive on StrCtx.

cc @kkent030315