Harmony RPC support on evm_increaseTime and evm_decreaseTime in test environments

[polymorpher]

These RPC calls are provided by ganache-cli (https://github.com/trufflesuite/ganache-cli). They are necessary for creating tests for smart contracts that are time-dependent, e.g. spending limit, lock out, last resort, etc.

This may be out of the scope for this SDK, but it would be extremely helpful to have this implemented sooner or later. Ideally there should something similar to ganache-cli, that generates a series of funded accounts in local environment, with ability to fund them at will.