add non-printf texstring(str)

[stevengj]

This adds a new API function char *texstring(const char *str), which is similar to stexprintf but doesn't do any printf formatting. (stexprintf is refactored to call texstring, so there isn't much duplication.)

It is useful to expose this lower-level functionality, e.g. for calling from higher-level languages that have their own printf-like functionality (e.g. Python f-strings) that may or may not use the same syntax. It's not equivalent to simply calling stexprintf with no arguments, because then you have to worry about escaping % characters. cc @Suavesito-Olimpiada

Also added a test script testsprintf.sh to test stexprintf — that function wasn't previously being tested; this involved adding an -S option to utftex so that it calls puts(stexprintf(...)) instead of texprintf(...).

(Will need a rebase/merge once #12 is merged.)

[bartp5]

I merged all your changes, so I suppose I can close this. Thanks!

[stevengj]

Thanks!