Add llvm_sizeof command

[andreistefanescu]

Let's add a llvm_sizeof command that returns the size (in bytes) of the given LLVMType. This should mirror the sizeof from C. The existing code that does that calculation in crucible_alloc is around https://github.com/GaloisInc/saw-script/blob/master/src/SAWScript/Crucible/LLVM/Builtins.hs#L1709.

[brianhuffman]

In #808 I implemented llvm_sizeof as a function that takes an LLVMModule argument. I'm not sure if this is exactly what you want; the alternative would have been a monadic function that runs in the CrucibleSetup monad: llvm_sizeof : LLVMType -> CrucibleSetup Int.

In any case, a simple pure type LLVMType -> Int is not possible, as we need an LLVM type context to know how to interpret alias types, struct padding, and alignment; and there is no such LLVM type context available in the saw-script TopLevel monad.