Modernized tooling and fixed four bugs. No API changes, and roots, proofs
and indexes are byte-identical to v0.2.0.

- go 1.21 is now the minimum (was 1.12)
- GitHub Actions replaces Travis
- VerifyContent now compares against the tree's merkle root
- sortAppend no longer writes into its callers' spare capacity
- RebuildTree no longer promotes the odd-count padding leaf to real content
- GetMerklePath tells left from right by node identity, not by hash