#ast appears to clobber node_ids

[marijnh]

Just filing this to make sure I don't forget it. The quasiquote test fails with monomorphization enabled, in a way that suggests a clash of node ids. If I wire ast_map to check that every id it encounters is new, it also complains for this test (and only for this test).

[pcwalton]

There was still an issue with node ID (actually node) reuse in #ast. Should be fixed now.