Optimization: use unions for representing RuntimeValue

[pepyakin]

Depends on #98

Currently, a RuntimeValue is represented by a rust enum.

As a refresher: A rust enum requires space for the variant tag plus the size of a payload of the largest variant, which also requires to be properly aligned. Thus, a RuntimeValue takes 8 bytes for the payload (for 64-bit wide values) and 8 bytes for the alignment (on x86_64).

We can shelve 8 bytes by removing a tag. We can achieve it by using C-like unions for representing runtime values internally. It is possible because after validation it is statically guaranteed that each operation will be used with operands of appropriate types (i.e., f32.* operators will always be used with f32 operands).

Shrinking RuntimeValue may potentially improve cache efficiency for the operand stack and remove branching to check operand types.