tock-registers: add trait-based manipulator methods to registers

Replace the individual implementations of common methods on
registers (e.g. get, set, read, modify) by traits which represent the
respective access level (Readable, Writeable, ReadWriteable).

This significantly reduces code duplication found in tock-registers by
providing default implementations for most methods, as well as
allowing to implement custom register types adhering to the
tock-registers interface. Furthermore, generic implementations of
peripherals can be designed to take trait objects instead of concrete
register types, to be agnostic over their actual register (e.g. some
register mapped to either a RISC-V CSR or a memory location).

The Readable, Writeable and ReadWriteable manipulation traits have
associated type parameters specifying the underlying integer type and
RegisterLongName. Associated type parameters are used, as every
monomorphized register over precise IntLike and RegisterLongName types
should have at most a single implementation of the respective
manipulation traits. If generic traits were used, multiple
manipulation traits could be implemented on a single monomorphized
register type.

Every readable register type implements Readable, which requires the
get-method to be implemented manually. Every writeable register type
implements Writeable, which requires the set-method to be implemented
manually. Registers which implement both Readable and Writeable, and
have the same associated IntLike and RegisterLongName types for both
of their Readable and Writeable implementations also implement
ReadWriteable. For Aliased registers (where read and write operations
correspond to different registers), ReadWriteable is therefore in
general not implemented.

Potential drawbacks of this approach include

- requiring to import Readable, Writeable and ReadWriteable to be able
  to use the respective methods

- having a standard interface, which does not allow deviation from

Signed-off-by: Leon Schuermann <[email protected]>

Author: lschuermann