feat(mine): Add an internal Zcash miner to Zebra #586

Job	Run time
Generate crates matrix	17s
Check crates matrix	7s
Build zebrad crate	27m 41s
Build zebra-utils crate	17m 19s
Build zebra-test crate	1m 37s
Build zebra-state crate	22m 38s
Build zebra-script crate	4m 43s
Build zebra-scan crate	19m 7s
Build zebra-rpc crate	22m 32s
Build zebra-node-services crate	3m 38s
Build zebra-network crate	7m 31s
Build zebra-consensus crate	17m 38s
Build zebra-chain crate	5m 15s
Build tower-fallback crate	1m 6s
Build tower-batch-control crate	12m 48s
Open or update issues for building crates individually failures	0s
	2h 43m 57s

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