Threads the fiat_{u64,u32}_backend features in the feature set

This allows the fiat backends introduced in
[curve25519-dalek/#342](dalek-cryptography/curve25519-dalek#342)
to be used from an ed25519 import without cumbersome overrides.

Cargo.toml

@@ -62,6 +62,8 @@ asm = ["sha2/asm"]
 legacy_compatibility = []
 u64_backend = ["curve25519-dalek/u64_backend"]
 u32_backend = ["curve25519-dalek/u32_backend"]
+fiat_u64_backend = ["curve25519-dalek/fiat_u64_backend"]
+fiat_u32_backend = ["curve25519-dalek/fiat_u32_backend"]
 simd_backend = ["curve25519-dalek/simd_backend"]
 
 [patch.crates-io]