Add ToStringMapStruct()

[albttx]

Add the possibility to cast a slice into a hashset (map[string]struct{}).

The struct{} element worth 0 byte, and it's really useful for checking if a key is present. here an article about the empty struct.

This pull request is link with https://github.com/spf13/viper/pull/437 to add this feature for parsing slice.

[CLAassistant]

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