cmd/yacc: memory allocation improvements

cmd/yacc: memory allocation improvements

Places a fixed size initial stack and the lval inside the parser
struct so that they are allocated together. Places $$char inside the
parser struct to avoid allocating the closure used in Lookahead().

Change-Id: I0de664a6d612279fdc3255633e2dff904030bc36
Reviewed-on: https://go-review.googlesource.com/16705
Reviewed-by: Russ Cox <rsc@golang.org>