windows fastcall (x64) call convention (#314)

* initial set of work for windows fastcall (x64) call convention

- call conventions: rename `fastcall` to `windows_fastcall`
- add initial set of filetests
- ensure arguments are written after the shadow space/store (offset-wise)
  The shadow space available before the arguments (range 0..32)
  is not used as spill space yet.

* address review feedback

lib/simplejit/src/lib.rs

@@ -23,6 +23,9 @@ extern crate errno;
 extern crate region;
 extern crate libc;
 
+#[cfg(target_os = "windows")]
+extern crate winapi;
+
 mod backend;
 mod memory;