Update and document Uint

Update and document Uint

This updates Uint classes to expose ZERO/ONE/MAX_VALUE and documents
their methods.

Caching is reworked to work on tunable statically-populated caches,
where Uint8 is always fully cached. This mirrors what is being done
for boxing purposes on java.lang.Byte, etc.

On top of the statically-populated caches, each type has a dedicated
Interner instance, which can be efficiently consulted using intern().

JIRA: YANGTOOLS-1018
Change-Id: Ic25b5ac685c920f7d82d588309a543f9a8eb43b7
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>