Patch: Implement Class.getSigners

[Tom Tromey]

Bryce> Shouldn't we get rid of _Jv_InitClassFields? There doesn't seem to be
Bryce> a need to set all these fields, because _Jv_AllocObject does that
Bryce> anyway.

We also call it from prims.cc:_Jv_InitPrimClass().  That call seems to
be actually needed.  There's probably a better way to do this.

I'd really like it if we got rid of the requirement to add a new
clause to the marker every time we add a field to Class.  Forgetting
this has caused bugs for us before.

Tom