introduce ethcore/state module

[rphmeier]

mostly just shuffling stuff around and fixing imports

bundles together:

• state.rs
• account.rs
• substate.rs

future work:

• arbitrary state backends
• persistent "master state"
• might put pod_* stuff in there

[coveralls]

Coverage decreased (-0.008%) to 86.964% when pulling a5cc40c on state_module into d279794 on master.

[NikVolf]

please use move files instead of copy-paste to preserve history and keep diff reasonable

https://githowto.com/moving_files

[coveralls]

Coverage decreased (-0.004%) to 87.018% when pulling 8c9f7a3 on state_module into 8159d74 on master.