for `~str`/`~[]`.
Note that `~self` still remains, since I forgot to add support for
`Box<self>` before the snapshot.
How to update your code:
* Instead of `~EXPR`, you should write `box EXPR`.
* Instead of `~TYPE`, you should write `Box<Type>`.
* Instead of `~PATTERN`, you should write `box PATTERN`.
[breaking-change]
Replaces existing tests for removed obsolete-syntax errors with tests
for the resulting regular errors, adds a test for each of the removed
parser errors to make sure that obsolete forms don't start working
again, removes some obsolete/superfluous tests that were now failing.
Deletes some amount of dead code in the parser, also includes some small
changes to parser error messages to accomodate new tests.
