Merge 'Refactor Json serialization to accommodate formatters for pret…

…ty printing' from Pedro Muniz

Json serialization logic was pulled from serde_json. Google's json5
serialization code was not flexible enough to allow for pretty printing
json, so I believe that the formatter design is a good layer to abstract
this logic. This refactor will trivially enable the implementation of
json_pretty function from sqlite. My other PR for json_quote, #763,
depends a tiny bit on a helper utility from the previous serialization
implementation. If this PR is considered first, I will change the code
in my other PR to account for this.

Reviewed-by: Diego Reis (@diegoreis42)
Reviewed-by: Kacper Madej (@madejejej)

Closes #771