COMPAT: Standardize method signatures

[gfyoung]

Currently, the parameter for values that one needs to insert into an array-like object such as DataFrame or Series is inconsistent across implementations. For example, base.py surfaces it as key, whereas series.py surfaces it as v.

This issue was identified while fixing an issue for v0.18.1 (xref #12413), but in the interests of maintaining backwards compatibility for a minor release, this change should be made as early as v0.19.0 with the following API comment:

"The signature for searchsorted has changed. It is now searchsorted(self, value, side='left', sorter=None)."

List of functions with signature inconsistencies:
searchsorted
repeat

@jreback
@jorisvandenbossche

[gfyoung]

In my quest to conquer fromnumeric.py incompatibilities, I have come across several other signature inconsistencies that I think also merit addressing. I've updated the original issue to reflect that.