SI-8086 addresses problem with calling Symbol.getter

Due to an unintended clash between `def getter: Symbol` (public reflection)
and `def getter(base: Symbol): Symbol` (internal reflection), the former
is uncallable from compiler internals and/or REPL’s power mode.

While both methods are guarded by compatibility constraints, we can’t
solve this problem radically and will have to live with it for some time
Thankfully, a replacement for `sym.getter` is quite approachable, namely:
`sym.getter(sym.owner)`.

In the meanwhile, I’ve deprecated the internal `getter` method by
renaming it to `getterIn`, also touching a nearby `superSymbol`. In the next
release we’ll be able to fix the name clash for good.