fix: Require mutable Context, as it should be

This spreads out transitively to many places and requires that
we use `check_call!` instead of `check_one_call` in a number of
places.

(cherry picked from commit 6bc31dcf206518a7be7f0ac6e773d5dfe25531ea)

rust/nix-expr/src/value.rs

@@ -73,7 +73,7 @@ impl Drop for Value {
 }
 impl Clone for Value {
     fn clone(&self) -> Self {
-        let context = Context::new();
+        let mut context = Context::new();
         context
             .check_one_call(|ctx_ptr| unsafe { raw::gc_incref(ctx_ptr, self.inner.as_ptr()) })
             .unwrap();