sync: Add RwLock::{try_read, try_write}

[pingw33n]

Closes #2284

[carllerche]

LGTM 👍 thanks!

[hawkw]

lgtm — thanks for adding loom tests!

n.b. that this change will conflict with #2445, which removes the ReleasingPermit type — whichever PR merges second will need to be rebased and updated. I think this branch should probably merge first.

[carllerche]

Looks like master needs to be merged.

[hawkw]

lgtm, thanks!

[Darksonn]

I merged master, and it broke with the following message:

error[E0308]: mismatched types
   --> tokio/src/sync/rwlock.rs:127:28
    |
127 |         lock.s.try_acquire(num_permits)?;
    |                            ^^^^^^^^^^^
    |                            |
    |                            expected `u32`, found `u16`
    |                            help: you can convert an `u16` to `u32`: `num_permits.into()`

[carllerche]

Closing due to inactivity. A new PR can be opened tracking master.