`ReadWriteLock` should subtype `Base.AbstractLock`

[AayushSabharwal]

This allows it to be used with Base.Lockable on 1.11+. It needs to implement Base.trylock to satisfy the interface.

[quinnj]

I think the main reason I didn't subtype originally is because I wasn't sure if lock/trylock should do a readlock or writelock by default

[AayushSabharwal]

Currently lock does a writelock, so it makes sense for trylock to do the same. I tried to write it in #43