More flexible operator overloading

[pcwalton]

The current system ties the various operators to a specific trait. This can be limited, as some uses for the operators might not fit that trait. For example, the return type might be different than expected, or the methods might be unsafe.

Postponed RFCs along these lines:

• Ad-hoc operator overloading, which removes traits altogether: Ad-hoc operator overloading #399
• Generic boolean operators, which extended return type for == and friends beyond bool: Generic boolean operators #1008