parser: tweaks to Insert AST
parser: %union type consolidation

Consolidated some types in %union since it's atually a struct
and not a union. Got rid of where, distinct and groupBy.
parser: consolidate comments into [][]byte
parser: got rid of some type assertions
parser: New intermediate types for better type safety
Merge branch 'parser'
Merge branch 'master' of github.com:youtube/vitess