Enable MW extensions (`__int128`,
`__vec2x32float__`) with `--mw-extensions`.
Disabled by default since these extensions
use `1` and `2` as type indicators, conflicting
with other demangling schemes (template args,
type names).
Includes extra logic to handle cases where
a function's class or template arguments
contain a type with a double underscore.
Fixes#2
Luke Street