Improve initial state.

[salls]

Use SpecialFillerMixin to improve reads of unmapped memory. Remove floating point support in fast mode. Store symbolic values in vex regs so a new symbolic value isn't created on those reads

[salls]

wait on this until https://github.com/angr/angr/pull/4664 is merged

[salls]

@Kyle-Kyle I fixed the commenting to clarify this is for uninitialized memory. I also tried with throwing an error on initialized memory access. However, with an error some more code was needed in other places and performance was not improved

[salls]

Also using the error on uninitialized reads breaks gadgets like this: mm0 is uninitialized so shows as an uninitialized read

p.factory.block(0x4421b5).pp() 4421b5 pmovmskb eax, mm0 4421b8 and eax, 0x80 4421bd ret

[Kyle-Kyle]

I thought you were going to hit the merge button. But apparently you didn't. Let me merge it then :)