Add support for StarkNet v0.8.0

[xJonathanLEI]

OK so as we're wrapping up our work on supporting #62, StarkNet v0.8.0 is already around the corner... Creating this issue to track the progress on supporting v0.8.0.

Can barely keep up :(

TODOs:

• [x] resolve breaking changes
• [x] add support for new gateway APIs
• [x] add support for paying transaction fees

[xJonathanLEI]

Looks like the StarkWare folks made quite some breaking changes. The ones I've discovered so far reading the code diff:

• some fields are now hex-encoded: calldata, constructor_calldata and more;
• the tx_id concept is gone. You can no longer query transactions or their receipts etc. by transaction ID; funny enough, get_transaction_hash_by_id and get_transaction_id_by_hash still work;
• as a result, the fieldtx_id is removed from TransactionFailureReason.

Most pre-0.8.0 clients should have been broken by this update. We probably need more tests.

[xJonathanLEI]

The library now fully supports StarkNet v0.8.0