Improve serializeJSON()

[fschaefer]

This commit tries to improve serializeJSON() to get closer to rfc4627.
It includes a more proper quoting of strings and output of integers
instead of floats where appropriate.

Purpose

At the moment the serializeString() method replaces only a subset of control characters that are allowed in JSON, hence stringify() may not serialize to valid JSON.

Goals

Add a better string quoting, closer to rfc4627.

Approach

Add a quote() method that walks through the chars of a string and replaces all escapable chars to their quoted "\uXXXX" counterpart.

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