feat: Make nixops4-eval async

A step toward handling the arrival of new data (stdin) with priority
over commands, avoiding roundtrips and re-evaluations.

(cherry picked from commit 8a2a5197886025caf35653001f76a4b209d8c9e4)

rust/nix-expr/src/value.rs

@@ -53,6 +53,7 @@ impl ValueType {
 pub struct Value {
     inner: NonNull<raw::Value>,
 }
+unsafe impl Send for Value {}
 impl Value {
     /// Take ownership of a new Value.
     ///