Univalue: Do not build JSON escape list at runtime

No need to waste startup time building something that can be done
at compile time.

This also resolves a clang++ warning originally reported in #4714,
univalue/univalue_write.cpp:33:12: warning: array subscript is of type 'char
escapes['"'] = "\\"";
^~~~

etc.

.gitignore

@@ -28,6 +28,8 @@ src/m4/ltsugar.m4
 src/m4/ltversion.m4
 src/m4/lt~obsolete.m4
 
+src/univalue/gen
+
 src/qt/*.moc
 src/qt/moc_*.cpp
 src/qt/forms/ui_*.h