Add BuiltinFnEnum

[ZibingZhang]

Overview

Creates an enum class BuiltinFnEnum which lets us easily reuse common function names.

Details

We can see this already being used in the function expander like so:

return ast.Call(
    func=ast.Name(id=constants.BuiltinFnName.SQRT.value, ctx=ast.Load()),
    args=[args_reduced],
)

As more function expanders are written, this will be used more.

References

Inspired from this conversation: #125 (comment)

Blocked by

None

[odashi]

Thanks!