Deprecate `!a == b`

[Timon Gehr]

A bug that crops up now and then in D is that someone negates `a == b` 
by prepending a `!`. The result is `!a == b`. This parses as `(!a) == b` 
and will often silently do the wrong thing because negation implies cast 
to `bool`, and `bool` can be compared with integral types and `enum` 
members.

I think it would be better for this to give a diagnostic and require 
explicit parentheses, similar to bitwise operators (where the operator 
precedence is unintuitive in the other direction).