Implement a base2.* to arith.* lowering

[KFAFSP]

For all closed base2 operations operating on built-in types, we should be able to delegate to arith, as long as we fix the rounding modes ourselves. There is quite some UB in arith.

• Fixed-point (includes fixed-point to integer)
• Floating-point ??? progress based on floating point redesign

[KFAFSP]

Here is an old candidate where I started doing this, now we should rely more on interface methods and Elaborator classes.

base2arith.txt

[KFAFSP]

My bad, forgot the lowering for fixed-point to integer.

LowerFixedPoint.txt