Put context object behind a ref rather than mut ref

This commit puts context object, i.e., the implementor of the
WASI snapshot, behind a reference `&self` rather than a mutable
reference `&mut self`. As suggested by @alexcrichton, this gives
the implementor the possibility to determine how it handles its
interior mutability.

tests/ints.rs

@@ -11,7 +11,7 @@ wiggle::from_witx!({
 impl_errno!(types::Errno);
 
 impl ints::Ints for WasiCtx {
-    fn cookie_cutter(&mut self, init_cookie: types::Cookie) -> Result<types::Bool, types::Errno> {
+    fn cookie_cutter(&self, init_cookie: types::Cookie) -> Result<types::Bool, types::Errno> {
         let res = if init_cookie == types::Cookie::START {
             types::Bool::True
         } else {