[sgen] Move Mono-specific wbarriers to `sgen-mono.c`.
[sgen] Move finalization callback stuff to `sgen-mono.c`.

Also introduce `sgen-client.h` which will eventually contain a spec of the
interface a client of SGen has to implement.
[sgen] Move some support functions into `sgen-gc.h`.
[sgen] Move ephemerons to client code.
[sgen] Move domain clearing into client code.
[sgen] Move array filling into client code.