Move hostcalls impl into separate module

src/macros.rs

@@ -0,0 +1,13 @@
+macro_rules! hostcalls {
+    ($(pub fn $name:ident($($arg:ident: $ty:ty,)*) -> $ret:ty;)*) => ($(
+            #[wasi_common_cbindgen::wasi_common_cbindgen]
+            pub fn $name($($arg: $ty,)*) -> $ret {
+                let ret = match crate::hostcalls_impl::$name($($arg,)*) {
+                    Ok(()) => crate::host::__WASI_ESUCCESS,
+                    Err(e) => e,
+                };
+
+                crate::hostcalls::return_enc_errno(ret)
+            }
+    )*)
+}