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.

crypto/schnorr/src/aggregate.rs

@@ -1,4 +1,7 @@
-use std::io::{self, Read, Write};
+use std_shims::{
+  vec::Vec,
+  io::{self, Read, Write},
+};
 
 use zeroize::Zeroize;