Support for generic types

[greyblake]

This is still work in progress.
It's supposed to address #133 and #130

To do

• Find a way to fix clippy warning using a reference to Cow is not recommended in

#[nutype(
    derive(Debug),
    validate(predicate = |s| s.len() >= 3),
)]
struct Clarabelle<'b>(Cow<'b, str>);

• Use internally a very unique lifetime identifier, instead of 'a. E.g. Ensure that this works:

#[nutype(
    derive(Debug),
    validate(predicate = |s| s.len() >= 3),
)]
struct Clarabelle<'a>(Cow<'a, str>);

• Make sure the example with Clarabelle can derive Display and other traits
• Make it possible to set trait boundaries:

#[nutype(
    sanitize(with = |v| { v.sort(); v }),
    derive(Debug)
)]
struct SortedVec<T: Ord>(Vec<T>)

NOTE: this will be handled in a separate story: #142