Move mtwt hygiene code to its own file

Also fixes some FIXME's
Fix FIXME 5074 in the mtwt hygiene code

which leads to a 2x speed gain for expansion pass.
Move SCTable and SyntaxContext_ from ast to ext::mtwt

to which they belong.
Specialize hasher for ext::mtwt lookup tables

which leads to another 2x speed gain for expansion pass.