refactor and clean up rwlock

This commit does a few things:

- Unify the lifetime annotations to all use `'rwlock` where some where
  using `'a`
- Unify the implementation blocks for the 4 different kinds of guards
- Adds documentation to the fields of each struct
- Aligns the fields of `RwLockWriteGuard` to be similar to the other 3

rename try_map to filter_map

Author: connortsui20