Reduce memory footprint

[appetrosyan]

The test introduced in #1784 demonstrated that creating a million accounts results in average memory usage of 5kiB per account. This is far in excess of what is acceptable for even modest performance requirements,

We should consider reducing the memory footprint significantly.

The proposed solution is to use Trait objects in lieu of statically-typed enum objects.

[e-ivkov]

This issue is especially relevant given https://github.com/hyperledger/iroha/issues/1777