[crypto] Check both backend features are not enabled simultaneously
[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 #4970, which made a feature control how we load dalek dependencies, and #5126 and following, which enable the new resolver and therefore help features have real mutually-exclusive semantics.

Closes: #5230
Approved by: metajack

libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core