reorg specs by fork and move ssz out to own folder

[djrtwo]

Modified core specs, ssz specs, and test specs/generators into a more manageable form. Notably:

• specs/ now has sub-directories grouped by fork (phase)
• SSZ is moved out to its own folder in anticipation of being released as its own spec
• test format specs and generators are grouped under tests/ and what used to be test_libs is moved into tests/core.

Made sure the build, tests, and generators run. Attempted to go through and fix all relative links within the repo. Very well could have missed something 😬

Yes, this is going to break external links to branch-based files. Links to files within releases will be fine. Discussed leaving a bunch of old files with redirects in them, but opted for this for cleanliness.

New Structure:

specs/
├── phase0/
│   ├── beacon-chain.md
│   ├── deposit-contract.md
│   ├── fork-choice.md
│   ├── p2p-interface.md
├── phase1/
│   ├── beacon-chain.md
│   ├── custody-game.md
│   └── shard-data-chains.md
│   └── light-client-sync.md
ssz/
├── simple-serialize.md
├── merkle-proofs.md
tests/
│   ├── core/
│   ├── formats/
│   └── generators/

[protolambda]

fork-dir-reorg really says it all. But things do look a lot better now, and better for phase1+ documents organization.

And /ssz can soon move out to some repo like ethereum/eth2.0-ssz (name TBD). For reference, the current more complete ssz spec draft is here: https://github.com/protolambda/eth2.0-ssz/ This draft will be updated with additional information to cover/reference partials, caching and merkle stores.