Rollup merge of #22596 - alexcrichton:fix-some-impls, r=huonw

 This commit removes many unnecessary `unsafe impl` blocks as well as pushing the
needed implementations to the lowest level possible. I noticed that the bounds
for `RwLock` are a little off when reviewing #22574 and wanted to ensure that we
had our story straight on these implementations.

diff --cc src/libstd/sync/condvar.rs
Simple merge

diff --cc src/libstd/sync/mutex.rs
Simple merge

diff --cc src/libstd/sync/rwlock.rs
Simple merge