zstring: fix encode rountrip for '\' as printable ASCII (#5120)

This fixes encode roundtripping for all printable ASCII characters.
In particular, this now leaves a plain '\' untouched by the
encoding logic, instead of converting it to escaped hex-digits.
It also adds unit testing covering this specific zstring encoding
property, in order to avoid future regressions.

src/test/main.cpp

@@ -215,6 +215,7 @@ int main(int argc, char ** argv) {
     TST(prime_generator);
     TST(permutation);
     TST(nlsat);
+    TST(zstring);
     if (test_all) return 0;
     TST(ext_numeral);
     TST(interval);