Add `@graphemes(..)`, `@bytes(..)` and `@chars(..)` (#338)
Add `@graphemes(..)`, `@bytes(..)` and `@chars(..)` (#338)

* Add grapheme, bytes, and char string -> array methods, with refactor

- Adds in the `@graphemes` `@chars`, and `@bytes` functions, which
create arrays of string variables corresponding to graphemes, unicode
scalars, and bytes respectively
- Add in start of select refactor

* Add tests for new string methods

bheesham/ion,bheesham/ion,redox-os/ion,jwbowen/ion,redox-os/ion,redox-os/ion,jwbowen/ion