* introduce grin_keychain, encapsulate derivation of secret_keys
* core compiles against keychain, tests don't run yet
* core tests are now passing against keychain
* wip - getting wallet working with keychain
* add util and keychain to travis test matrix
* basic test around key derivation
* Integrate PMMR and its persistent backend with the Chain
* Chain can set tree roots; PMMR backend discard
* Check spent and prune for each input in new block
* Handling of forks by rewinding the state
* More PMMR tests and fixes, mostly around rewind
* Rewrite get_unspent to use the sumtrees, fix remaining compilation issues
* use head_header in add_to_memory_pool
* add COINBASE_MATURITY const to consensus
* add coinbase maturity (wip) validaton rule to validate_block
* add coinbase maturity check to validate_block
* map errors in adapters - specific errors still wip
* reworked so adapter translates chain errors to pool errors (core errors not required)
* add test for spending immature coinbase in memory pool
* wip - add test to cover spending coinbase output in chain.process_block
* added test coverage around process_block - we have a problem with coinbase output commitments
* add a comment on the failing test
* process_block will now fail validation if we attempt to spend coinbase that has not yet matured
(remember to use a new reward_key for every block). test coverage in place to verify this
