ast.go:
- rename Comments -> CommentGroup (less confusion)
- change all comments/docs to be *CommentGroup

filter.go:
- do not remove unassociated comments from program as part
  of export filtering (they are needed by doc.go for BUG comments)

scanner.go:
- exclude '\n' from //-style comments

parser.go:
- rewrote collection of comments: now all collected comments
  are *ast.CommentGroups
- clarified distinction between leading and trailing comments
- fixed a bug in comment collection (parseGenDecl);
  corresponding test case is in printer/testdata
- extra documentation

doc.go:
- collect BUG comments
- corresponding fix for parser bug in comment collection

comment.go:
- simplified regex

printer.go:
- adjust comment printing to new representation

printer_test.go, testdata/*:
- enable printing of doc comments
- extended tests

package.html, package.txt:
- added Bugs section

gofmt:
- enable printing of doc comments

R=rsc
DELTA=339  (126 added, 38 deleted, 175 changed)
OCL=31403
CL=31416