Uint31 support.
Int31 literals in native compiler.
Had to split Nativelambda in two files because of Retroknowledge
dependencies.
Partial support for open terms in int31.
Full support for int31 values in native compiler.
Readback for int31 values from native compiler.
Machine arithmetic operations for native compiler.

This completes the port of the native compiler to retroknowledge.
However, some testing and optimizations are still to be done.
Int31 decompilation in native compiler was still partial. Now fixed.
Optimizing Int31 support in native compiler, by not tagging
applications of I31 constructor as lazy.