Constify conversion traits (part 1)
This is the first part of rust-lang/rust#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature:
* `From`
* `Into`
* `TryFrom`
* `TryInto`
* `FromStr`
* `AsRef`
* `AsMut`
* `Borrow`
* `BorrowMut`
* `Deref`
* `DerefMut`
There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`:
* `ByteStr::new` (unstable under `bstr` feature)
* `OsStr::new`
* `Path::new`
Those which directly use `Into`:
* `Result::into_ok`
* `Result::into_err`
And those which use `Deref` and `DerefMut`:
* `Pin::as_ref`
* `Pin::as_mut`
* `Pin::as_deref_mut`
* `Option::as_deref`
* `Option::as_deref_mut`
* `Result::as_deref`
* `Result::as_deref_mut`
(note: the `Option` and `Result` methods were suggested by ``@npmccallum`` initially as rust-lang/rust#146101)
The parts which are missing from this PR are:
* Anything that involves heap-allocated types
* Making any method const than the ones listed above
* Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO)
r? ``@tgross35`` (who mostly already reviewed this)
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