gc: Remove redundant private protection from imports
gc: Update Copyright years
gc: Group extern (C) declarations to avoid repetition
gc: Rename modules to make more sense

Remove the `gc` prefix from `gcbits` and `gcstats` (we already have the
`gc` package to know where they belong).

Rename `gcx` to `gc` because it holds the main GC implementation and
`gc` to `proxy` because it basically holds a GC proxy.

As a side effect, `src/rt/memory.d` needs to be fixed to import the GC
interface from `core.memory` instead of accessing directly to the GC
implementation.