Inconsistent naming of MPTree entries

[davidyuk]

https://github.com/aeternity/protocol/blob/master/serializations.md#contracts-state-tree

Contracts' state tree => contracts
Contract Calls' state tree => calls
Channels' state tree => channels
Nameservice's state tree => mtree (?)
Oracles' state tree => otree (?)
Accounts' state tree => accounts