Officially rename '%' from mod() into rem() in traits and docs

[Kimundi]

Rusts % operator mimics C and C++ in that it's not modulo but remainder. This behavior should probably not be changed, however the documentation and naming convention should make it clear that it's the remainder operation.

[graydon]

Did #5990 fix this?

[thestinger]

The % operator is now correctly called rem since that's the fast operation. The slower wrapper can be implemented on top of it and correctly called modulo since the method isn't taken anymore.