- printing of expressions: put spaces only where "needed"
- printing of import statements: no double indentation if there are no renames
- print labels on separate lines
- added extra test files

go/ast:
- unified basic literal nodes and as a result deleted duplicated code
- added initial code to track scopes (not fully used yet)

replaces CL 34553

R=rsc
DELTA=881  (579 added, 223 deleted, 79 changed)
OCL=34623
CL=34651