Use Uint64 for 'length' argument

Use Uint64 for 'length' argument

RFC6020/7950 place an upper bound on the integral size of length
argument -- explictly calling out that an implementation does not
have to support size beyond unsigned 64bit.

Take advantage of this to lower our memory footprint, by using our
yang.common.Uint64 class instead of java.math.BigInteger. Also
use Uint64.intern() to minimize the number of actual instances we
create.

JIRA: YANGTOOLS-1206
Change-Id: Id112279eb9632d06a80e444d16bd5c825e4ba4ff
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>