MacroRepImpls

Implementations of the various .rep/.repX overloads. The most common and simple overloads are implemented as macros for performance, while the more complex/general cases are left as normal methods to avoid code bloat and allow the use of default/named arguments (which don't work in macros due to https://github.com/scala/bug/issues/5920).

Even the normal method overloads are manually-specialized to some extent for various sorts of inputs as a best-effort attempt ot minimize branching in the hot paths.