LibSyntax is the only remaining user of UTF-32 in the code base. Let's
use UTF-8 here.
Bonus: The tests added here for non-ASCII sources actually used to
crash the old UTF-32 implementation.
This first pass only applies to the following two cases:
- Public functions returning a view type into an object they own
- Public ctors storing a view type
This catches a grand total of one (1) issue, which is fixed in
the previous commit.
