Add const constructors to RwLock, Notify, and Semaphore.

[steffahn]

Referring to the types in tokio::sync.
Also add const to new for the remaining atomic integers in src/loom (except for 64bit which is impossible) and to UnsafeCell.

Motivation and Solution

Builds upon previous work in #2790.
Closes #2756.

This PR adds const_new to sync::Notify, sync::RwLock, and sync::Semaphore, in addition to the previously added one for sync::Mutex as they can all be reasonably be used in static variables and this saves the need for (and overhead of) something like lazy_static in the case that parking_lot already is used internally anyways.

Also, upon encountering the problem of missing const for AtomicU8 in src/loom (used in Notify), I decided (also for consistency) to add const to all the constructors for the remaining types in that directory, whereas #2790 only added const to new for AtomicUSize.

[Darksonn]

Please try to avoid force-pushing, as it makes reading the change-history difficult for me to read.

[steffahn]

Please try to avoid force-pushing, as it makes reading the change-history difficult for me to read.

I was going to stop force-pushing as soon as a review started or more than a typo-sized mistake was being fixed.