[RFC 61] Add Modulo Operator (and floored division)

[mfelsche]

Implementation of RFC 61.

This PR adds the following methods to the number classes hierarchy and adds the implementation to each stdlib implementation (U8, U16, ... I8, I16, ..., F32, F64):

Real

• fld
• mod

Integer

• fld_unsafe

• fld_partial

• fldc

• mod_unsafe

• mod_partial

• modc

FloatingPoint

• fld_unsafe
• mod_unsafe

[mfelsche]

Also i did not implement %% as compiler intrinsic as it was easier to implement in pony and afaik results in the same LLVM IR, thus same machine code.

Also, outside the scope of this RFC, but what about \ as operator for fld?

[mfelsche]

Some properties mentioned in the RFC have been verified here (when run against ponyc built from this branch): https://github.com/mfelsche/stdlib-properties/blob/master/stdlib-properties/main.pony#L82-L100

[SeanTAllen]

@mfelsche RFC includes updates to the tutorial. I don't want to merge this until that is also available as the tutorial changes were part of the RFC that was accepted.

[mfelsche]

tutorial PR is here: ponylang/pony-tutorial#347