feat: make dag-pb spec compliant

[dignifiedquire]

Closes #168, #139

Depends on

• [x] https://github.com/multiformats/rust-multihash/pull/252
• [x] https://github.com/multiformats/rust-cid/pull/129

[dignifiedquire]

@vmx this should be pretty ready now

[vmx]

I dared to merge my "less globs import" change, in return, I made Clippy even happier.

[vmx]

I have a look at the outstanding failures.