- converted more of AST and parser to use interfaces and explicit
structs for individual Go constructs (can be replaced now with
interface calls such that the parser becomes AST structure
independent, as suggested by rsc)
- added more tests (find all .go files under GOROOT)
- (temporarily) lost html links for identifiers when generating
html output
- TODO: lots of cleanups

R=r
OCL=25518
CL=25518