This previously only worked in certain circumstances. In particular,
assignments such as `match[1] = ...` or even just `match[1]` would fail
to parse correctly.
Fixing this turned out to be less trivial than anticipated. Consider the
fact that
```
match [1]: case (...)
```
can either look the start of a `match` statement, or it could be a type
ascription, ascribing the value of `case(...)` (a call) to the item at
index 1 of `match`.
To fix this, then, we give `match` the identifier and `match` the
statement the same precendence in the grammar, and additionally also
mark a conflict between `match_statement` and `primary_expression`. This
causes the conflict to be resolved dynamically, and seems to do the
right thing in all cases.
