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' [-Wchar-subscripts]
escapes['"'] = "\\"";
^~~~

etc.
I think it's useful to refer to `gen.cpp` in the generated file as well. After all, someone maintaining this will stumble on this file first.