Check for small subgroup components when using the EdwardsPoint group for the OPRF

A malicious attacker sending a small point could lead the server / user to leak private information.

The check avoiding mixed order points (rather than just small) is prohibitively expensive, and I don't know how an attacker would extract any data from that => we focus on a small order check.

Fixes #34.

Cargo.lock

@@ -1,5 +1,11 @@
 # This file is automatically @generated by Cargo.
 # It is not intended for manual editing.
+[[package]]
+name = "anyhow"
+version = "1.0.32"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b602bfe940d21c130f3895acd65221e8a61270debe89d628b9cb4e3ccb8569b"
+
 [[package]]
 name = "atty"
 version = "0.2.14"
@@ -451,6 +457,7 @@ checksum = "624a8340c38c1b80fd549087862da4ba43e08858af025b236e509b6649fc13d5"
 name = "opaque-ke"
 version = "0.1.0"
 dependencies = [
+ "anyhow",
  "base64",
  "criterion",
  "curve25519-dalek",