Rename mod and related operators to rem

[mfelsche]

as our mod is actually calculating the remainder (results have the sign of the dividend, e.g. -5 % 2 = -1),
not the modulo (result have the sign of the divident, e.g. -5 % 2 = 1).

A proper modulo operator will be added in a later PR.

[jemc]

This looks good, but I'd like to have a corresponding tutorial PR ready to merge that fixes the terminology there as well.

[mfelsche]

This PR: ponylang/pony-tutorial#323 incorporates the mod->rem changes. I merged both together as the partial arithmetic PR contains lots of mentions of the modulo operator and thus changed it there.

[SeanTAllen]

@mfelsche can you add release notes to this PR please?

[mfelsche]

Release Notes Draft

This change renames the % operation from mod, standing for Modulo, to rem, standing for Remainder. The reason is that % in Pony was actually computing the remainder, not the modulo. The difference is obvious when handling signed integers:

The result of the Modulo operation always has the sign of the divisor, e.g.: -5 mod 2 = 1.
The result of the Remainder operation always has the sign of the dividend, e.g.: -5 rem 2 = -1.

Pony does the latter. We are working on adding a proper Modulo operator later on.

This is a breaking change, if you used one the mod function on Integers or implemented your own mod operator on your data types.