Add multiple lock methods for RWLocks

[Areskiko]

Currently only one lock method can be provided, and it is expected give mutable access to the underlying data. While pure read locks don't make much sense, RWLocks are quite common, so this is an important feature to implement.

[Areskiko]

The idea is to change how the lock method is specified from

#[lock_method = "lock()"]

to

#[lock_metod("write_lock()", "read_lock()")]

where it is expected that the first method gives mutable access, and the second gives immutable access. The latter one should obviously be allowed to be omitted if it is not applicable.