Fix/jumps

[guillep]

Several fixes in Jumps:

• use unsigned conditional comparisons
• use Jump: instead of genJump
• do not Use genMoveConstant if we know it's not an oop
• better support for extensions

To make that work, this PR makes two main changes:

• better type system collaborating with slang to detect signed/unsigned conditionals
• better support for jit compile-time expressions, separating constants from complex expressions