`SvcParams.String()` used to iterate over keys in a random order, which
meant that calling `ServiceBinding.RR()` could return different `Data`
for identical records. This commit sorts the keys in lexicographic
order, making the result deterministic.

Furthermore, since it seems useful for code to assume that it can safely
roundtrip between `Record.RR()` and `RR.Parse()` and that `Record.RR()`
is deterministic, I've now documented this. These are both already the
case, but a formal guarantee should make external usage easier.

Related to #189.

* Implement e2e testing suite + dummy in-memory implementation

* Log records for troubleshooting

* Fix e2e tests for interface-based Records and separate provider types

* Fix code review comments: TTL, cleanup, no IP copy-paste

* Add multiple record types and a way to exclude some, simplify API, update README

* Fix README formatting

* Restore ZoneLister optionality

* Use RFC 2606 compliant DNS names

* Update skipping API to use SkipRRTypes

* Rename e2e to libdnstest, dummy to example

…192)