Remove MatcherPos::stack.

`parse_tt` needs a way to get from within submatchers make to the
enclosing submatchers. Currently it has two distinct mechanisms for
this:
- `Delimited` submatchers use `MatcherPos::stack` to record stuff about
  the parent (and further back ancestors).
- `Sequence` submatchers use `MatcherPosSequence::parent` to point to
  the parent matcher position.

Having two mechanisms is really confusing, and it took me a long time to
understand all this.

This commit eliminates `MatcherPos::stack`, and changes `Delimited`
submatchers to use the same mechanism as sequence submatchers. That
mechanism is also changed a bit: instead of storing the entire parent
`MatcherPos`, we now only store the necessary parts from the parent
`MatcherPos`.

Overall this is a small performance win, with the positives outweighing
the negatives, but it's mostly for clarity.

compiler/rustc_expand/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(associated_type_bounds)]
 #![feature(associated_type_defaults)]
+#![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(crate_visibility_modifier)]
 #![feature(decl_macro)]