Support global variables

I don't really like this shared parsing here but it doesn't work anyway if we move to "static".  parse_let_stmt() is already pretty small; it seems okay to duplicate (but replacing LetExpr with StaticStmt or whatever).  If we *really* wanted to, we could split the (maybe Type, maybe Pat) parsing to a generic function and pass (Static, StaticItem) or (Let, LetExpr), but that seems excessive.