Always use global `bind` and `flip` in `infixbindl` and `infixbindr`.

This also eliminates the only two cases where a `StructMixIn` is created without a location.
Move simple variable-related stuff out of `store_top`.

Also, use it in `import` instead of `store_top`. This eliminates the only remaining `StructMixIn` created outside of `parse`.
Remove unnecessary `location`-assigning methods.