Describe the bug
Hi there, in json_string_builder-inl.h:157 the array is not using the correct json escape mapping, its being escaped as \\x00XX instead of \\u00XX which is invalid, it's also not doing upper case conversions.

parsing a document produced by builder::string_builder with dom::parser when the documents contains escaped characters isn't possible.

The control char \b is also not escaped as \\b but as \\x0008

To Reproduce
There are no test in the codebase to check the behavior of escaping.

simjson release
Commit 8fff575