symbolic: Deduplicate mkInitialRegVal across projects

[langston-barrett]

Apparently, it's common to make an initial struct of completely symbolic registers:

macaw/symbolic/src/Data/Macaw/Symbolic/Testing.hs

Line 290 in 429df8e

mkInitialRegVal :: (CB.IsSymInterface sym, MT.HasRepr (MC.ArchReg arch) MT.TypeRepr)

https://github.com/GaloisInc/pate/blob/c43542818be7780264d8a2552bfeba1cffba2902/src/Pate/Verification/InlineCallee.hs#L88

https://github.com/GaloisInc/ambient-verifier/blob/eab04abb9750825a25ec0cbe0379add63f05f6c6/src/Ambient/Verifier/SymbolicExecution.hs#L172

https://github.com/GaloisInc/saw-script/blob/47a7632c973df3fdb2d32268edc92948fc150f2d/src/SAWScript/X86Spec.hs#L453

We should probably export this kind of functionality from macaw-symbolic, to prevent reinventing the wheel. Ideally, this could have some sort of tie-in with the solution to #430, such that the initial register state is reasonably suitable for executing actual machine code.

This does kind of toe the line of what should be included in macaw-symbolic. Certainly, one can imagine clients that would not want to set all registers to fully symbolic values. However, it appears to be relatively common in practice. I think it's not unreasonable to include helpers for setting macaw-symbolic up "in the normal way", and allow clients with more specific needs to use lower-level APIs to achieve what they want. We could also consider collecting this sort of functionality in a separate library, so that not every dependency would have to pay the price of additional compile time.

[langston-barrett]

See also freshValue in macaw-x86-symbolic:

macaw/x86_symbolic/src/Data/Macaw/X86/Symbolic.hs

Line 261 in 429df8e

freshValue ::

[RyanGlScott]

And for even more prior art, see freshCrucibleConstant in macaw-symbolic:

macaw/symbolic/src/Data/Macaw/Symbolic.hs

Lines 1281 to 1288 in 429df8e

	-- | Create a fresh Crucible constant based on the supplied Macaw 'M.TypeRepr'.
	freshCrucibleConstant ::
	forall sym tp.
	IsSymInterface sym =>
	sym ->
	SolverSymbol ->
	M.TypeRepr tp ->
	IO (C.RegValue sym (ToCrucibleType tp))