Change shadowing mechanism of extension methods for on par implicit class behavior

[benhutchison]

@Ichoran

Status quo: extension methods effectively have no namespace. They clobber each other willy-nilly like everything without a namespace always does, getting quadratically bad the more they are used.

A “me too” to this experience report. @Ichoran’s description aligns with my experiences trying to apply extension methods. They are prone to name collisions. It can be a minefield.

Most recently, on an upgrade from 3.3.0-RC2 → 3.3.0-RC3, I started seeing new error msgs: Note that overloaded methods must all be defined in the same group of toplevel definitions. Previously working extension methods in two different compilation units stopped compiling.