Api cleanup

[fjarri]

• added a HRAC::SIZE constant and moved ADDRESS_SIZE to Address::SIZE
• added HRAC.from_bytes() in Python bindings to match WASM bindings
• renamed RevocationOrder::verify_signature() into verify() and made it return a Result<(Address, EncryptedKeyFrag)>. Correspondingly, made RevocationOrder.staking_provider_address private.
• made MetadataResponse::verify() and ReencryptionResponse::verify() return a Result instead of Option
• added VerificationError for the use of verification methods above

[codecov-commenter]

Codecov Report

Merging #14 (4d075ad) into master (cb15d48) will not change coverage.
The diff coverage is 0.00%.

@@          Coverage Diff           @@
##           master     #14   +/-   ##
======================================
  Coverage    0.00%   0.00%           
======================================
  Files          12      12           
  Lines         428     434    +6     
======================================
- Misses        428     434    +6     

Impacted Files	Coverage Δ
nucypher-core/src/address.rs	0.00% <0.00%> (ø)
nucypher-core/src/hrac.rs	0.00% <0.00%> (ø)
nucypher-core/src/key_frag.rs	0.00% <0.00%> (ø)
nucypher-core/src/node_metadata.rs	0.00% <0.00%> (ø)
nucypher-core/src/reencryption.rs	0.00% <0.00%> (ø)
nucypher-core/src/revocation_order.rs	0.00% <0.00%> (ø)

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