Refactor syntax, suggested by kibit
Extract missing-sym-patterns to avoid repetition

Visually diffing the regular expressions in #'missing-sym-name was
difficult because they were not aligned to the same column.

Commit 56b5b6b5624673b859c878ae851161f50bcd9f5d added '\+' to one
pattern for vars, but not to the others. This broke with the history of
missing-sym-name, which has used the same pattern for matching vars and
namespaces since its introduction.

We restore this symmetry in this commit for the sake of uniformity.

While we're touching this pattern, add a test for finding
non-alphanumeric vars.
Find special-case token "/" during regrow

Although / is used as the namespace qualifier, the Clojure reader does
treat a lone / as a valid token for the sake of the division function.

Therefore namespaces can also declare their own vars named /, so we
should attempt to find them.

Note that clojure.core// is also special cased; my.ns// is invalid.