new string library

[Ben Pfaff]

I've revised, and largely rewritten, the string library in
libpspp/str.[ch].  The new version is much more powerful.  It has
already allowed some code to be significantly simplified and made
easier to read.  I'm expecting that it should allow similar
improvements across much of the rest of the code base, especially
in situations where strings are parsed.

Function naming has changed quite a bit.  The new names are much
more uniform than the old ones were, so this should be an
improvement also.

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