Lazy converter instantiation performance gotcha

Converters are instantiated (and then memoized) lazily on first use. Since this is a rather heavy operation, it can influence timing-sensitive code as [recently observed in Aleph's test suite](https://github.com/clj-commons/aleph/issues/626). This is bit of a gotcha which [was even brought up in a past issue already](https://github.com/clj-commons/byte-streams/issues/10). That resulted in [the introduction of a `precache-conversions` API](https://github.com/clj-commons/byte-streams/commit/bea74d6927b9763d5ec0a026ad602bed9f8e28a4). However, it [later got removed again](https://github.com/clj-commons/byte-streams/commit/863e311b55d336488bf0345c1994e74f4e3416f8) without further explanation. In any case, the fact that this issue came up again indicates that such an API would still be desirable.