Added FmtConst impl for String and Vec<Primitive>

[jean-airoldie]

This allows the user to user owned types for dynamic code generation.

Here is a example where this is required:

let mut bldr = phf_codegen::Map::new();
// In this case symbol_ids is a HashMap<String, u32> whose content is only known
// at runtime.
for (symbol, id) in symbol_ids {
    bldr.entry(symbol, &id.to_string());
}
let display = bldr.build().to_string();

Previously the user would have to leak the String to get a &'static str.

I also ran rustfmt, which explains some of the diff.

[s3bk]

yes please. just ran into this while attempting to update.

[neandrake]

I ran into the lack of FmtConst implementation for String while updating from 0.7 to 0.8. I confirmed that with a 1-line change to add the implementation and my previous output from 0.7 matches the new output from 0.8

delegate_debug!(String);

[neandrake]

For consistency I would probably move this one up above under the delegate_debug!(str); line then have this next grouping be for Vec<>(). Would it make sense to have Vec<String> as well?

[abonander]

delegate_debug!() is not correct for Vec<_> because it prints an array literal.

[JohnTitor]

I've merged the support for String hence the remaining task is how we deal with Vec<T>.