exp/ssa: reimplement logic for field selection.

The previous approach desugared the ast.SelectorExpr
to make implicit field selections explicit.  But:
1) it was clunky since it required allocating temporary
   syntax trees.
2) it was not thread-safe since it required poking
   types into the shared type map for the new ASTs.
3) the desugared syntax had no place to represent the
   package lexically enclosing each implicit field
   selection, so it was as if they all occurred in the
   same package as the explicit field selection.
   This meant unexported field names changed meaning.

This CL does what I should have done all along: just
generate the SSA instructions directly from the original
AST and the promoted field information.

Also:
- add logStack util for paired start/end log messages.
  Useful for debugging crashes.

R=gri
CC=golang-dev
https://codereview.appspot.com/7395052
cmd/vet: fix printf test for unsafe Pointer
And fix test. Pointer to unsafe.Pointer tests nothing important...
Also identify the incorrect type: go/types.Type is a Stringer.

Also fix a couple of incorrect format verbs found by new printf checker,
now that we can run it on more files.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7385051