Make CurrentAssertionLevel thread local

[fingolfin]

Right now, CurrentAssertionLevel is global. I think this is problematic in HPC-GAP (ping @rbehrends @stevelinton). So I suggest we make it thread local instead.

The easiest way to do that would be to remove it as a GAP variable, and instead put it into the kernel, as a member of the thread local GAPState. That would also require moving SetAssertionLevel and AssertionLevel from the library to the kernel, but that's easy. I can't think of any downsides?

[rbehrends]

The only potential problem I can see is that code that depends on the current gvar (and bypasses AssertionLevel() and SetAssertionLevel() would break. But I don't know any code that would do that? (The documentation describes CurrentAssertionLevel as unpublished, but one cannot always rely on user code adhering to that.)

Other than that, that looks like a sensible change. It would probably also make checking the assertion level in GAP code slightly faster.