Encode strings into AST

DIFFBASE=57

DO NOT MERGE

This PR is for discussion.
It encodes strings directly into AST, achieving two goals:

  1.  make AST more self-sufficient - at some stage we should be able to ship compiled AST to the remote box and execute it
  2.  decreases space taken by string encoding. This PR drops generated AST size for minified jquery from 250k down to 110k, making AST size almost the size of a source (93k).