Allow modes on self

[eholk]

It'd be nice to pass self by move, or at least by copy (whatever + mode is). This would make my new vector addition in the library code faster since we could avoid always copying self.

[pcwalton]

This will be addressed by the removal of argument modes, plus max/min classes.

[catamorphism]

@pcwalton Is there an issue open for max/min classes yet?

[pcwalton]

@catamorphism Not yet, still ironing out the design.

[eholk]

This is now part of #2794, correct?

[msullivan]

Not really part of #2794. This is now the related "explicit self type" proposal.