Factor out runtime string support

[reviewed by David]

This patch factors out the runtime string support into
chpl-string.[ch].

While I was there, I removed four unused #defines.  The ones for
NANSTRING, NEGINFSTRING, and POSINFSTRING are unused in chpltypes.c,
and they are already defined in chplcast.c (and only used there).  The
other one was chpl_emptyCstring which should have been removed with
r23558.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23621 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Add Coverity Scan model file.

Model files help eliminate false positives by giving coverity hints about halts
and memory (de)allocation. The only model, for now, helps coverity understand
that INT_FATAL and USR_FATAL abort execution.

r: sungeun

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23622 3a8e244f-b0f2-452b-bcba-4c88e055c3ca
Move coverity scripts into util/devel/test/coverity/.

r: sungeun

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23623 3a8e244f-b0f2-452b-bcba-4c88e055c3ca