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/lib.rs

@@ -0,0 +1,24 @@
+#![cfg_attr(not(feature = "std"), no_std)]
+
+#[cfg(not(feature = "std"))]
+#[allow(unused_imports)]
+#[doc(hidden)]
+#[macro_use]
+pub extern crate alloc;
+
+pub mod collections;
+pub mod io;
+
+pub mod str {
+  #[cfg(not(feature = "std"))]
+  pub use alloc::str::*;
+  #[cfg(feature = "std")]
+  pub use std::str::*;
+}
+
+pub mod vec {
+  #[cfg(not(feature = "std"))]
+  pub use alloc::vec::*;
+  #[cfg(feature = "std")]
+  pub use std::vec::*;
+}