Fix STable claiming.

With this, t/serialization/01-basic.t now fully passes.
scsetcode needs to set code object's SC

Fixes remaining failures in t/serialization/02-types.t.
Ensure context_object is NULL and marked.

Avoids one segfault in closure serialization.
Better error reporting.
Pass correct object; fixes 03-closures.t.
Add serialization tests to selftest.
Fix serialization string heap code-gen.

Just nabbed a fix from JVM version.
Add and map getscdesc op.
A couple more deserialization code-gen fixes.

This gets --target=mbc to produce MAST for a small example; next fail
is in bytecode generation.
Implement DLL symbols
Merge branch 'master' into dll

Conflicts:
	lib/MAST/Ops.nqp
	src/core/ops.c
	src/core/ops.h