Handle unary minus

[lfairy]

-a can be translated to 0.wrapping_sub(a), but that causes a type error as the 0 is ambiguous.

One workaround is 0i32.wrapping_sub(a), where i32 is replaced with the type of a. But that requires us to know the type of the expression in advance, which is impossible with macros as they run before type inference.

I think we should simply disallow unary minus altogether.

[lfairy]

One solution could be to use Zero::zero(), but I think that still causes ambiguity.

[lfairy]

I just realized we can translate -x to x.wrapping_mul(-1) instead. This side-steps the ambiguity problem because the literal is now on the right-hand side. I'll try this solution tomorrow.