Auto merge of rust-lang#26141 - eddyb:ast_map, r=nikomatsakis

Gets libsyntax one step closer to running on stable (see rust-lang#24518).
Closes rust-lang#24757, erickt's previous attempt at this.

1.0.0 release

1.0.0-alpha.2 release

1.0.0-alpha release

0.12.0 release

0.11.0 release

0.10 release

0.9 release

0.8 release