In this pull request, we propose a solution for the empty taint location issue involving XMLHttpRequest.response, or better, string literals in dynamically evaluated tainted scripts. Hence, this pull request fixes #321 partially, as fetch.text() and fetch.json() taint operations are still flawed by this issue for other reasons.

First, we add nlohmann/json (taint/json.hpp), a popular and maintained library for JSON parsing and serialization. We decided to add it because Firefox internals only provide JSONWriter, not JSONReader, and the JSON features provided by the JS engine require a JSContext*, which is not available everywhere we need it, including at allocate time in ParserAtom (?).
Then, in Taint.h we declare the functions ParseStringTaint and SerializeStringTaint to parse and serialize, respectively, the whole StringTaint data structure as a simplified JSON string. Their implementation in Taint.cpp is based on nlohmann/json.
Finally, in Taint.h we replace the functions ParseTaint with ParseStringTaintForE2E and serializeStringtaint with SerializeStringTaintForE2E for the end-to-end tainting. We rewrite the functions using nlohmann/json and thus discarding the custom JSON parser, while preserving the expected behavior of these functions.

Other

• Formatted String.cpp and jstaint.{cpp,h}.
• Fixed naming and typing for TaintOperation::source and TaintOperation::native.
• Minor naming fixes.