go/parser: unify parsing of const and var declarations

go/parser: unify parsing of const and var declarations

The AST representation is already identical. Making the
code (nearly) identical in the parser reduces code size
and ensures that the ast.ValueSpec nodes have the same
values (specifically, iota). This in turn permits the
sharing of much of the respective code in the typechecker.

While at it: type functions work now, so use them.

R=r
CC=golang-dev
https://golang.org/cl/6624047