Simple extern FFI

[rurban]

See https://github.com/perl11/potion/wiki/FFI and p2 branch ffi

• calling convention with different ABI (syntax and code, no win64, no stdcall yet)
• either precompile each ffi proto to a signature translation wrapper, or translate the sigs at the call ext part in the vm, similar to filling in defaults.
• return type declaration syntax. like an optional extern int sleep(sec=N).
  or extern sleep(sec=N) => N
• call it (vm and jit)
• move some of compile-time parts to the VM (OP_EXTERN dlsym). currently we can only call into already loaded libs, or load a lib at compile-time (as in p2 BEGIN{})

aggregate types and header parser later

[rurban]

work is done in branch extern. arguments are not yet converted