- renamed Program -> SourceFile
- added Package node representing the AST for an entire package
- added filter function to create a source file mimicking the
  interface of an entire package

parser:
- parser entry to parse entire packages
- unified naming of parser entry points
- factored out entry points into new file (interface.go)

gofmt:
- extended to accept single .go files, and package paths:
  gofmt file.go	     	    // formatting of a single file
  gofmt -x file.go	    // interface of a single file
  gofmt -x ./MyPackage	    // interface of a local package
  gofmt -x math		    // interface of a $GOROOT relative package

Various adjustments in dependent files, documentation.

R=rsc
DELTA=634  (369 added, 153 deleted, 112 changed)
OCL=31743
CL=31748