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/Cargo.toml

@@ -16,6 +16,9 @@ region = "0.2.0"
 libc = { version = "0.2.40", default-features = false }
 errno = "0.2.3"
 
+[target.'cfg(target_os = "windows")'.dependencies]
+winapi = { version = "0.3", features = ["winbase", "memoryapi"] }
+
 [features]
 default = ["std"]
 std = ["libc/use_std", "cretonne-codegen/std", "cretonne-module/std", "cretonne-native/std"]