Refactor to idiomatic Result/Option patterns (#25)

This:
- introduces a small [thiserror](https://github.com/dtolnay/thiserror)-powered enum to improve ProofVerifyError's messages,
- refactors point decompression errors into a variant of that enum, thereby suppressing the panics which occur when decompresison fails.
- folds other panics into the Error cases of their enclosing `Result` return

src/group.rs

@@ -1,9 +1,25 @@
+use super::errors::ProofVerifyError;
 use super::scalar::{Scalar, ScalarBytes, ScalarBytesFromScalar};
 use core::borrow::Borrow;
 use core::ops::{Mul, MulAssign};
 
 pub type GroupElement = curve25519_dalek::ristretto::RistrettoPoint;
 pub type CompressedGroup = curve25519_dalek::ristretto::CompressedRistretto;
+
+pub trait CompressedGroupExt {
+  type Group;
+  fn unpack(&self) -> Result<Self::Group, ProofVerifyError>;
+}
+
+impl CompressedGroupExt for CompressedGroup {
+  type Group = curve25519_dalek::ristretto::RistrettoPoint;
+  fn unpack(&self) -> Result<Self::Group, ProofVerifyError> {
+    self
+      .decompress()
+      .ok_or_else(|| ProofVerifyError::DecompressionError(self.to_bytes()))
+  }
+}
+
 pub const GROUP_BASEPOINT_COMPRESSED: CompressedGroup =
   curve25519_dalek::constants::RISTRETTO_BASEPOINT_COMPRESSED;