Support UTF8 sequence length

I think it would be good to expose a free function from `bstr` that exposes some decoding-specific information about what a given byte means in the context of utf8.

Accessing this info is low level, but has various use cases -- some examples include finding a place to start parsing from given an index, finding a legal cutoff position if you need to truncate a buffer... Etc. (Let me know if you want more cases, I feel like I run into it a fair bit when working with partially invalid utf8).

Specifically, something like this:

```rust
// If `b` indicates the start of a utf8 sequnence boundary,
// returns `Some(sequence_len)`. Returns `None` for all other cases.
pub fn utf8_sequence_len(b: u8) -> Option;
```

Or... Maybe. I'd kinda like to distinguish between valid-but-not-leading and always-invalid bytes. Returning an enum maybe? Thoughts and bikeshedding welcome, I think in practice this would be useful, but also wanted to keep the things small and simple.

---

That said, I do feel strongly that this should not be methods on byteslice like `ByteSlice::is_char_boundary(&self, index: usize) -> bool` and `ByteSlice::utf8_sequence_len(&self, index: usize) -> Option` (mentioning mostly because I suggested these in #42) -- I think those two would be very confusing in practice:

- `ByteSlice::is_char_boundary` would have to return different results from `str::is_char_boundary` even for a fully utf8 byte slice (example: index == len). Having the caller get the byte in question avoids this issue. (Renaming it doesn't even really solve this problem -- still seems like it could cause confusion if 0/len are not conidered boundaries).

- `ByteSlice::utf8_sequence_len(&self, idx)` could behave too many ways -- specifically IDK if it only reads `self[idx]` or if it considers other bytes nearby (e.g. if it's not a leading byte). Making it a top level function only taking a `u8` removes this ambiguity -- reasonably only one thing it could do

Begin by reviewing the bstr public API and existing UTF-8 handling; no file or test is named. Resolve whether the API distinguishes leading, continuation, and always-invalid bytes, then define focused coverage for the selected behavior and return shape.

Written by the indexing model from the issue text.