Refactor Data.toString (#2197)

Provides a more intuitive implementation of toString for Data.
Utilizes the fact that the compiler plugin provides names earlier than
Chisel had in the past so we can accurately guess the name of
signals even in the currently elaborating module.

Co-authored-by: Megan Wachs <megan@sifive.com>
Co-authored-by: Jack Koenig <jack.koenig3@gmail.com>