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/native/src/lib.rs

@@ -37,7 +37,7 @@ pub fn builders() -> Result<(settings::Builder, isa::Builder), &'static str> {
     if cfg!(any(unix, target_os = "nebulet")) {
         flag_builder.set("call_conv", "system_v").unwrap();
     } else if cfg!(windows) {
-        flag_builder.set("call_conv", "fastcall").unwrap();
+        flag_builder.set("call_conv", "windows_fastcall").unwrap();
     } else {
         return Err("unrecognized environment");
     }