[mypyc] Generate faster code for bool comparisons and arithmetic

[JukkaL]

Generate specialized, efficient IR for various operations on bools. These are covered:

• Bool comparisons
• Mixed bool/integer comparisons
• Bool arithmetic (binary and unary)
• Mixed bool/integer arithmetic and bitwise ops

Mixed operations where the left operand is a bool and the right operand is a native int still have some unnecessary conversions between native int and int. This would be a bit trickier to fix and is seems rare, so it doesn't seem urgent to fix this.

Fixes mypyc/mypyc#968.