[crypto] Check both backend features are not enabled simultaneously

This would typically happen with somebody compiling with `--features "vanilla"` rather than `--features "vanilla" --no-default-features`.

This is a follow-up to aptos-labs#4970, which made a feature control how we load dalek dependencies, and aptos-labs#5126 and following, which enable the new resolver and therefore help features have real mutually-exclusive semantics.

Closes: aptos-labs#5230
Approved by: metajack

crypto/crypto/src/lib.rs

@@ -43,3 +43,9 @@ compile_error!(
     "no dalek arithmetic backend cargo feature enabled! \
      please enable one of: fiat, vanilla"
 );
+
+#[cfg(all(feature = "fiat", feature = "vanilla"))]
+compile_error!(
+    "at most one dalek arithmetic backend cargo feature should be enabled! \
+     please enable one of: fiat, vanilla"
+);