crucible-mir: Split out the Misc CastKind into more precise constructors

[RyanGlScott]

Currently, the Misc CastKind is a catch-all for several unrelated CastKinds, as can be seen in this code:

crucible/crucible-mir/src/Mir/JSON.hs

Lines 405 to 416 in c310fcb

	-- TODO: actually plumb this information through if it is relevant
	-- instead of using Misc
	Just (String "PointerExposeAddress") -> pure Misc
	Just (String "PointerFromExposedAddress") -> pure Misc
	Just (String "DynStar") -> pure Misc
	Just (String "IntToInt") -> pure Misc
	Just (String "FloatToInt") -> pure Misc
	Just (String "FloatToFloat") -> pure Misc
	Just (String "IntToFloat") -> pure Misc
	Just (String "PtrToPtr") -> pure Misc
	Just (String "FnPtrToPtr") -> pure Misc
	x -> fail ("bad CastKind: " ++ show x)

As a result, it is difficult to distinguish between all of these CastKinds in Mir.Trans.evalCast'. I propose that we delete the Misc constructor and replace it with more precise constructors that reflect the structure of the CastKind enum in the rustc API. This will require some detective work to figure out which existing uses of Misc are covered by which CastKind variants.