Reworked json export.

The json export prints formatted json, which takes a lot of
additional time, however the result is only consumed by other tools
and not meant for human reading.

This commit implements several small changes in order to speedup
the json export:

* Removal of usage of the Format Module
* Replacing `fprintf` calls by calls to function that print
  directly, such as `output_string`, etc.
* Replacing list of all instruction names by a set of all
  instructions