This allows optional subsystems to not needlessly reserve too much
system memory and it also reduces offsets bookkeeping.

so that it isn't picked up automatically at boot. Only when (c<$) is
called do we override (c<). This fixes tricky boot problems.

Let's revisit the relinking approach, with LITA this time.
This reverts commit 6652125d47.

It seems that xcomp can't manage to reliably compile core+ units.
Let's limit the role of xcomp to z80c+icore.

I'm getting a little desperate here. I'll revisit the link.fs approach
with a new idea I had...

Replace its usages with direct RAM+ offsets. The (sysv) mechanism was
incompatible with cross-compilation of a full interpreter.

It's not needed anymore now that the boot binary is built in one pass.

... instead of building boot.fs, and then z80c.fs/icore.fs in separate
passes. This strengten xcomp by making it support 2 completely separate
dicts (that is, the X dict is *not* connected to the system dict. The chain
now stops at EXIT def in boot.fs).

We can now cross-compile core.fs.

It was a dead end. my new tentative solution is xcomp.

Memory mapping is a dead end too, solution has to be at a higher
level. xcomp is my new approach.

This makes IMMEDIATE affected by memory mappings.

This way, it can be affected by memory mappers.

How did this ever work?

This was a dead end.

Why didn't I think of adding it before?