Evaluate using string wrapper types for slugs and petnames

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In fixing #108 we introduced a few transparent wrapper types for strings so that we could more explicitly type what kinds of strings are expected in some cases and cut down on the amount of repeated documentation for those strings.

In #379 , we aligned on the possibility that this could be a useful pattern for two new stringly types: slugs and petnames.

We should evaluate potentially making wrappers for these types. The wrappers could be implemented to force validation during initialization, thereby cutting down on the need to guard all usages of slug/petname stringly types against possible misuse.