This PR prepares chains specs for _native-runtime-free_  world.

This PR has following changes:
- `substrate`:
  - adds support for:
- JSON based `GenesisConfig` to `ChainSpec` allowing interaction with
runtime `GenesisBuilder` API.
- interacting with arbitrary runtime wasm blob to[
`chain-spec-builder`](https://github.com/paritytech/substrate/blob/3ef576eaeb3f42610e85daecc464961cf1295570/bin/utils/chain-spec-builder/src/lib.rs#L46)
command line util,
- removes
[`code`](https://github.com/paritytech/substrate/blob/3ef576eaeb3f42610e85daecc464961cf1295570/frame/system/src/lib.rs#L660)
from `system_pallet`
  - adds `code` to the `ChainSpec`
- deprecates
[`ChainSpec::from_genesis`](https://github.com/paritytech/substrate/blob/3ef576eaeb3f42610e85daecc464961cf1295570/client/chain-spec/src/chain_spec.rs#L263),
but also changes the signature of this method extending it with `code`
argument.
[`ChainSpec::builder()`](https://githu...