Fix curve25519-dalek backend selection.

Author: hdevalence

Cargo.toml

@@ -23,7 +23,8 @@ rand = "0.7"
 serde = "1"
 serde_derive = "1"
 thiserror = "1"
-curve25519-dalek = { version = "2", features = ["serde"] }
+# Disable default features to deselect a backend, then select one below
+curve25519-dalek = { version = "2", default-features = false, features = ["serde", "std"] }
 
 [dev-dependencies]
 bincode = "1"
@@ -33,3 +34,7 @@ sha2 = "0.8"
 nightly = ["curve25519-dalek/nightly"]
 debug-transcript = ["merlin/debug-transcript"]
 bench = []
+default = ["u64_backend"]
+u32_backend = ["curve25519-dalek/u32_backend"]
+u64_backend = ["curve25519-dalek/u64_backend"]
+simd_backend = ["curve25519-dalek/simd_backend"]