Rename str_def (mlstring)

[dnezam]

As discussed with @myreen and @tanyongkiam

• HOL: Rename str to chr_to_str
• HOL: Add and use overload toString for chr_to_str
• CakeML Basis: Rename str to toString

Another thing this could enable is that parameters/variables called strng could be renamed back to str (assuming type and term parsing don't interact in unforseen ways), which was introduced in 680922b