Allow map keys that are effectively strings

[dtolnay]

Fixes XAMPPRocky/tokei#83. cc @Aaronepower

[oli-obk]

If we are serializing unit variants, shouldn't we also serialize unit structs and units ( the latter as the empty string)? Does roundtripping work/ should it?

[dtolnay]

Unit structs and units are different because they serialize as null which is not a string. Unit variants serialize as "VariantName" which is a string. I don't think it is obvious to expect null in a map key to behave like an empty string (I know, even though we deserialize null to String as an empty string). Also who uses maps with unit keys?

Round tripping works, I added a test.