This commit adds `into_inner` methods to the `Mutex`, `RwLock`,
`spin::RwLock`, and `spin::Mutex` types, allowing the inner value to be
extracted from the lock when the lock is owned exclusively. This seemed
like a nice thing to add.

This commit adds `get_mut` methods to `Mutex`, `RwLock`, `spin::Mutex`,
and `spin::RwLock`, allowing mutable access without locking when the
lock itself can be accessed mutably. These are identical to the
`std::sync::Mutex::get_mut` and `std::sync::RwLock::get_mut` methods.

This commit adds `Default` impls to `Mutex`, `RwLock`, and `spin::Mutex`
where `T: Default`, allowing locked data to be constructed through
`Default` without needing `Mutex::new(Default::default())` or similar.

An implementation of `Default` for `spin::RwLock` was not added, because
I already did that in #472.