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/io.rs

@@ -0,0 +1,85 @@
+#[cfg(feature = "std")]
+pub use std::io::*;
+
+#[cfg(not(feature = "std"))]
+mod shims {
+  use core::fmt::{Debug, Formatter};
+  use alloc::{boxed::Box, vec::Vec};
+
+  #[derive(Clone, Copy, PartialEq, Eq, Debug)]
+  pub enum ErrorKind {
+    UnexpectedEof,
+    Other,
+  }
+
+  pub struct Error {
+    kind: ErrorKind,
+    error: Box<dyn Send + Sync>,
+  }
+
+  impl Debug for Error {
+    fn fmt(&self, fmt: &mut Formatter<'_>) -> core::result::Result<(), core::fmt::Error> {
+      fmt.debug_struct("Error").field("kind", &self.kind).finish_non_exhaustive()
+    }
+  }
+
+  impl Error {
+    pub fn new<E: 'static + Send + Sync>(kind: ErrorKind, error: E) -> Error {
+      Error { kind, error: Box::new(error) }
+    }
+
+    pub fn kind(&self) -> ErrorKind {
+      self.kind
+    }
+
+    pub fn into_inner(self) -> Option<Box<dyn Send + Sync>> {
+      Some(self.error)
+    }
+  }
+
+  pub type Result<T> = core::result::Result<T, Error>;
+
+  pub trait Read {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+      let read = self.read(buf)?;
+      if read != buf.len() {
+        Err(Error::new(ErrorKind::UnexpectedEof, "reader ran out of bytes"))?;
+      }
+      Ok(())
+    }
+  }
+
+  impl Read for &[u8] {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+      let mut read = 0;
+      if self.len() < buf.len() {
+        read = self.len();
+      }
+      buf[.. read].copy_from_slice(&self[.. read]);
+      *self = &self[read ..];
+      Ok(read)
+    }
+  }
+
+  pub trait Write {
+    fn write(&mut self, buf: &[u8]) -> Result<usize>;
+    fn write_all(&mut self, buf: &[u8]) -> Result<()> {
+      if self.write(buf)? != buf.len() {
+        Err(Error::new(ErrorKind::UnexpectedEof, "writer ran out of bytes"))?;
+      }
+      Ok(())
+    }
+  }
+
+  impl Write for Vec<u8> {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+      self.extend(buf);
+      Ok(buf.len())
+    }
+  }
+}
+
+#[cfg(not(feature = "std"))]
+pub use shims::*;