MIR-borrowck: dereferences are printed wrapped in parens

[pnkfelix]

MIR-borrowck: In error diagnostics, derefencing expressions are printed with parentheses around them.

For example, for borrowck-borrow-mut-object-twice.rs, AST-borrowck reports:

cannot borrow `*x` as mutable more than once at a time (Ast)

while MIR-borrowck reports:

cannot borrow `(*x)` as mutable more than once at a time (Mir)

(This is hopefully relatively simple to resolve; is there any case where we need the parentheses, given that we already elide dereferences when printing field accessor expressions?)

[nikomatsakis]

@zilbuz are you looking at this?

[zilbuz]

@nikomatsakis Yes I am :)