Fix performance degradation in commands that describe key bindings

Fix performance degradation in commands that describe key bindings

* src/keymap.c (syms_of_keymap)
<describe-bindings-check-shadowing-in-ranges>: New variable.
(describe_vector): Check shadowing of consecutive keys only if
'describe-bindings-check-shadowing-in-ranges' is non-nil.  Remove
redundant second loop when VECTOR is a char-table.  Improve
comments.  Patch by Stefan Kangas <stefan@marxist.se>.  (Bug#45379)

* test/src/keymap-tests.el
(help--describe-vector/bug-9293-one-shadowed-in-range): Adapt the
test case for the new variable.