struct parse_params: trivial packing

* parse.y (struct parse_params): shrink to 320 to 304 bytes on 64-bit

Some elements may be removed/resized as well, but not as important
as reducing more-frequently allocated structures.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46954 b2dd03c8-39d4-4d8f-98ff-823fe69b080e