Add as_ref and as_mut methods for Bound

[glittershark]

Add as_ref and as_mut method for std::ops::range::Bound, patterned off
of the methods of the same name on Option.

I'm not quite sure what the process is for introducing new feature gates (this is my first contribution) so I've left these ungated, but happy to do whatever is necessary to gate them.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

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[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling unwind v0.0.0 (/checkout/library/unwind)
error: associated function has missing stability attribute
   --> library/core/src/ops/range.rs:684:5
    |
684 | /     pub fn as_ref(&self) -> Bound<&T> {
685 | |         match *self {
686 | |             Included(ref x) => Included(x),
687 | |             Excluded(ref x) => Excluded(x),
688 | |             Unbounded => Unbounded,
690 | |     }
    | |_____^

error: associated function has missing stability attribute
error: associated function has missing stability attribute
   --> library/core/src/ops/range.rs:694:5
    |
694 | /     pub fn as_mut(&mut self) -> Bound<&mut T> {
695 | |         match *self {
696 | |             Included(ref mut x) => Included(x),
697 | |             Excluded(ref mut x) => Excluded(x),
698 | |             Unbounded => Unbounded,
700 | |     }
    | |_____^

error: aborting due to 2 previous errors
error: aborting due to 2 previous errors

error: could not compile `core`

To learn more, run the command again with --verbose.
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "--message-format" "json-render-diagnostics"
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap check
Build completed unsuccessfully in 0:01:18

[dtolnay]

This needs an unstable attribute (https://rustc-dev-guide.rust-lang.org/stability.html) and a Library Tracking Issue (https://github.com/rust-lang/rust/issues/new/choose).

[glittershark]

@dtolnay done, I think. Let me know if I did it right

[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit eb72dc5 has been approved by dtolnay