Stop walking all of AST on every pass

[marijnh]

Most passes aren't interested in, for example, types, yet the default visitor runs through all type nodes on every walk anyway.

We could provide several variants of default_visitor(), which descend into different subsets of the AST, and make sure each pass uses the right variant.

[boggle]

Nice, this should speed things up. How are you doing it? I see a bitv that describes which passes the visitor should enter.

[marijnh]

There's no bitv. A pass has function values for entering each kind of node. You can pass no-op functions for nodes that don't contain anything interesting.