pattern bindings can be mutable

[nikomatsakis]

I just realized that my recent fix opened up another hole in borrowck. I believe that pattern bindings over mutable memory will be considered mutable but are only guaranteed to be const. This discrepancy should be fixed (also probably need a test case ensuring that the data is not considered immutable)

[nikomatsakis]

I think this is no longer relevant with the introduction of ref bindings.