Add conformance testing

[bgins]

Summary

Problem

We don't test against a canonical set of fixtures to check conformance with the UCAN specification.

Impact

There may be issues with the implementation that we are not aware of.

Solution

Write conformance tests that check against canonical fixtures.

ETA: 2023-09-01

Note that we can't do this quite yet because the fixtures are still being written. A proposed set of test fixtures is under review: ucan-wg/conformance-tests#13