Change type of block number and timestamp to uint64_t

[gumb0]

Currently it's int64_t, but some tests have timestamps that fit only in uint64_t.

Related TODOs:
https://github.com/ethereum/evmone/blob/a9d5bfe1791c4577a9c8a17a1790ed81fe4b3f78/test/blockchaintest/blockchaintest_runner.cpp#L148
https://github.com/ethereum/evmone/blob/68714ad3d5717d7bb8083e1365aecc62e179d51b/test/statetest/statetest_loader.cpp#L53

This requires changes in EVMC (evmc_tx_context)