Refactor cli structure

[AntoineRondelet]

• Refactored the structure of the CLIs

• Renamed zeth_debug CLI to zeth_helper

• Re-arranged the client code:

  • zeth becomes core (the cli is called zeth, enhance having a zeth folder suggested it was the code of the cli while it actually was the "core"/"backend" of the client)
  • test becomes tests
  • commands becomes cli
  • test_commands is now split in:
    - test_commands: scripts used to carry out "integration" tests and deploy testing contracts/generate data for the tests
    - helper: CLI allowing to carry out non-zeth related things. This is not strictly used for tests, but rather to ease the use of Zeth on an Ethereum-like blockchain by exposing some useful functionalities

• core, api, helper, cli are bundled into a zeth package

[AntoineRondelet]

Supersedes #252

[dtebbs]

Just a few comments up front:

zeth becomes core (the cli is called zeth, enhance having a zeth folder suggested it was the code of the cli while it actually was the "core"/"backend" of the client)

It woudl be great if you could put everything in a zeth subdirectory, like we do in zecale. This makes the hierarchy deeper unfortunately (so we have client/zeth/core, client/zeth/commands etc), but seems to be standard practice for python and it makes it much easier to use the code as a dependency (we can import zeth.core instead of import core)
I hit conflicts with this kind of thing in several projects that use zeth, and meant to add this structure to zeth. If you are renaming modules than now might be a good time to do it.

commands becomes zeth_cli

So this can become zeth/cli, etc... (The scripts entry in setup.py can refer to zeth/cli/zeth of course)

test becomes unit_tests

I think test is a reserved name, known to the unit test framework and the package manager (dependencies cannot import the test module, I think), so it might be better to keep this as it is.