Add the `base2.fixed.*` exact operations

[KFAFSP]

For all operations permissible on fixed-point types, including those exclusive to them (e.g., rescale), an exact operation should exist. These infer their result types from the operand types.

• base2.fixed.rescale
• base2.fixed.align
• base2.fixed.cmp ??? only if there is a more efficient algorithm then align + cmp
• base2.fixed.add
• base2.fixed.sub
• base2.fixed.mul
• base2.fixed.div
• base2.fixed.mod ??? underspecified

Check FixedPointInterpreter for more details.