Add llvmcall

This adds `llvmcall` as described in #4042. This is the simple version of this intrinsic. 
I have another version that basically takes functions in place of the IR and the various type declarations, allowing you to integrate any llvm based compiler. I'm a lot less sure about that interface than I am about this one, but I figured I'd mention it in this context anyway, since it's another point of discussion that's fairly similar (in particular, how do we efficiently do inline IR,C,ASM,D,whatever)?
small typo: sceleton -> skeleton