Merge: JSON: optimize writing strings

Merge: JSON: optimize writing strings

Changes how strings are serialized to JSON by grouping non-escaped characters in a single call to write. So if a string has no escape characters, it is written as is. This replaces the old behavior where there was a call to write for each character.

This PR addresses the same issue as #2367 and #2371 but it targets JSON string writing only. The best results are obtained when this PR is merged with #2371. On my machine, parsing and writing `magic.json`  takes the following wall clock time depending on the branch:

* master: 14s
* #2367 (char cache): 6.2s
* #2371 (buffer): 4.0s
* this PR: 4.8s
* this PR + #2371: 3.6s

Pull-Request: #2372