Change how `fn(&T)` parses

[nikomatsakis]

Right now, fn(&T) parses as fn(&v: T) rather than fn(v: &T). This should be changed so that modes require explicit variable names. We would also of course then need to update the various legacy fn() types that appear in rustc and elsewhere.

[Vincent-Belliard]

I did the patch to disable modes with unamed arguments. However, we have now a quite confusing error message:

error: mismatched types: expected `fn&(&int)` but found `fn(&int)` (expected argument mode ++, but found &)

So I modify the argument dump to display:

error: mismatched types: expected `fn&(&int)` but found `fn(&:int)` (expected argument mode ++, but found &)

I wonder if this display could become a new syntax to specify modes for unamed arguments (that is adding a colon between mode and type).
I you believe so, just tell me and I will add this syntax.

[nikomatsakis]

Oh, good point (regarding the conversion of the type to a string). However, I don't think a new syntax is needed, because modes will be removed shortly.

[catamorphism]

I take it this can be closed, since Vincent's patch is in incoming.