Removed language-level BigIntegers in SOM

SOM BigIntegers removed like in TruffleSOM to enable purely VM internal representation changes between normal ints (RPython int) and arbitrary precision integers (RPython rbigint).

The SOM level is always using Integer, to have separate primitives, the implementation moved to the VM-level classes to have them polymorphic.
Also adapted the Double implementation accordingly.
Another semantic change is that double conversion does not lead to a naive resend, but does it directly.

SOM library updated accordingly.

Signed-off-by: Stefan Marr <git@stefan-marr.de>