Add no_std support to transcript, dalek-ff-group, ed448, ciphersuite, multiexp, schnorr, and monero-generators

transcript, dalek-ff-group, ed449, and ciphersuite are all usable with no_std
alone. The rest additionally require alloc.

Part of #279.

common/std-shims/src/collections.rs

@@ -0,0 +1,7 @@
+#[cfg(feature = "std")]
+pub use std::collections::*;
+
+#[cfg(not(feature = "std"))]
+pub use alloc::collections::*;
+#[cfg(not(feature = "std"))]
+pub use hashbrown::{HashSet, HashMap};