Besides code cleanup, this commit changes the sanitization logic to
avoid numeric digits altogether. Previously, they had been allowed at
the end of a label name, but this is susceptible to a ZZT parser bug:
`#foo` matches `:foo2` because ZZT allows a prefix match if the next
character is a digit.
This change allows qualified references to local labels from other
sections: you can do `#send section.local` and it will take you to the
appropriate `:.local` label. When `section.local` is ambiguous due to
sections/locals sharing the same names, the reference resolves to the
local in the first such section.