Refactor type-conversion of input parameter to INTERVAL into an
auxiliary function COERCE-INTERVAL-DESIGNATOR.

Fix test definition not to pass single integer to INTERVAL:INSERT
since it expects an INTERVAL or a (CONS x x).
Speed-up implementation of NODE-FIND-ALL by:

  a) not trying to concatenate results if not necessary,
  b) using APPEND rather than CONCATENATE because CONCATENATE
     works on any sequences.
Micro-optimize INTERVAL-INTERSECTS by declaiming it inline and canonicalizing
the return value of its function parameter to a true boolean.
Add new function INTERVAL:FIND-ANY.

It's like FIND-ALL but stops the search when it finds an interval
in the tree that intersects with the passed interval.

The function is useful if you want to find out if there's any
interval in the tree intersecting with a specific interval.
s/(DEFSYSTEM/(ASDF:DEFSYSTEM/g in cl-interval.asd and cl-interval-test.asd.