utility functions to deal with Library-of-Congress call numbers

 Library::CallNumber::LC is mostly designed to do call number normalization,
 with the following goals:
 .
  * The normalized call numbers are comparable with each other, for proper
    sorting
  * The normalized call number is a short as possible, so left-anchored
    wildcard searches are possible (e.g., searching on "A11*" should give you
    all the A11 call numbers)
  * A range defined by start_of_range and end_of_range should be correct,
    assuming that the string given for the end of the range is, in fact, a
    left prefix
 .
 That last point needs some explanation. The idea is that if someone gives a
 range of, say, A-AZ, what they really mean is A - AZ9999.99. The end_of_range
 method pads the given call number out to three cutters if need be. There is no
 attempt to make end_of_range normalization correspond to anything in real life.