Spec tests for v0.6.1

Updates ssz_static tests to use offsets (SOS-style) in serialization.

Spec tests for v0.6.0

Start of new testing:

• Standardized general format, see ethereum/eth2.0-specs/specs/test_formats
• Git LFS for handling larger test vectors
• Introduce ssz_static testing: fully cover SSZ containers of phase 0.
• Port over previous tests to new consistent format
• Constants can be loaded on compile time, see ethereum/eth2.0-specs/configs/constant_presets
• Base test-generator introduced, more tests to be build with it: test generator documentation
• POC deposits tests. More state-transition tests to be introduced, after generalizing between spec-pytests and test-generators better.

Note: serialized in ssz_static is on normal (non-SOS) SSZ: v0.6.1 introduces a test-vector update here.