Support for numbers as `Feature.ID`

[DmitriyVTitov]

This PR provides backward compatibility although with that requirement I think we cannot preserve ID type information after Marshal - Unmarshal (number would become string). So this is a kind of workaround. All tests pass and my own project works with this code.

[twpayne]

This is great - thank you.

There are a couple of minor tidy-ups needed, but I'll do these in a separate PR.

[DmitriyVTitov]

@twpayne Great! Could you create a new tag pls?

[twpayne]

Created v1.5.0.