Remove custom Serde implementation (#44)

* Serialize `BlindedElement` as `GenericArray`

* Don't hold input

* Remove allocations from `VerifiableClient::batch_finalize`

* Remove all allocation from serialization

* Remove required `alloc` support.

* Fix accidental usage of 1.57 API

* Let `VerifiableClient::batch_finalize` return a concrete type

* Simplify de-serialization

* Remove custom Serde implementation

src/tests/mod.rs

@@ -5,6 +5,7 @@
 // License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 // of this source tree.
 
+#[cfg(feature = "alloc")]
 mod mock_rng;
 mod parser;
 mod voprf_test_vectors;