feat(tonic): implement From<Code> for i32

`Code` is already trivial to convert into an `i32` however that isn't
immediately obvious by looking at the docs. This has tripped me up
before.

Implementing `From<Code>` for `i32` should hopefully make it a bit more
obvious.

tonic/src/status.rs

@@ -754,6 +754,13 @@ impl From<i32> for Code {
     }
 }
 
+impl From<Code> for i32 {
+    #[inline]
+    fn from(code: Code) -> i32 {
+        code as i32
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;