* UnixTimestampConverter should now obey a maximum value
This change prevents an issue where the converter would be unable to
handle obscenely large timestamp values - which are actually quite
common on Discord.
OptionalConverter had to be rewritten to support checking whether or not
an InnerConverter returned an Optional. The perf impacts from this
_shouldn't_ be too bad, as types without a custom parser (which should
be the majority of Optionals in the lib) will bypass the type-check.
* optimizations on OptionalConverter
bfaa6fc97a