This PR reduces the size of a `FormatElement` from 56 to 32 bytes by:

* Using a `Box<str>` to store a dynamic token's string as it doesn't need to be mutable (safes 8 bytes for the `capacity`)
* Only storing the start position for a `DynamicToken` because the length isn't used by the printer
* Change the semantics of `verbatim_ranges` to store the ranges in the **formatted** string. The ranges in the formatted output should be sufficient for debugging and it allows to resolve the `String` rather than having to store it on the `FormatElement` (range can be computed in the printer).

This improves the formatter's performance by about 10%