Negative fixtures framework (Go & JS), and negative fixtures for dag-pb!

[rvagg]

Finally this closes #80.

• Negative fixtures framework in JS & Go for running decode() operations and checking failure conditions - blocks are defined in hex within the fixture docs.
• Negative fixtures framework in JS & Go for running encode() operations and checking failure conditions - the JSON files containing the fixtures define a data model form in an existing codec that can be decoded (currently only dag-json, just plainly presented inside the JSON, but this will likely need to be extended to present a hex form for other codecs so there's optionality), then that data model form is passed through the codec's encoder and error is checked
• Docs
• Fixtures for encode and decode for dag-pb, extracted from the common tests that I've built up for both current JS & Go implementations.

No Rust yet; but I'd really love to get these in and expanded: dag-pb mainly for Iroh and dag-cbor mainly for FVM.

• I need to "fix" the JS dag-pb's error messages to match what I have here, it highlighted some inconsistencies and a bit of sloppiness, will do a PR over there following this.

[rvagg]

ipld/js-dag-pb#69 - will be a semver-major because we're messing with error messages.