add `Range` support for `{:in, choices}` type

[dcrck]

Ranges implement Enumerable and support in, so you should be able to use them with the {:in, choices} type.

For example, to check if a value is a byte:

schema = [decimals: [type: {:in, 0..255}]]

This commit adds that support.

[josevalim]

Looks good, thanks! We just need a test and I think we are good to go!

/cc @whatyouhide good to you too?

[dcrck]

added some tests