In several places, we check for a control character with something like
char c;
[...]
if (c >= 0 and c <= 0x1F)
[...]
When char is signed (e.g. amd64) this is fine, but when char is unsigned
by default (e.g. arm32 and arm64) this generates warnings about the
tautologous check that an unsigned value is non-negative.
Write as
if ((unsigned char) c <= 0x1F)
[...]
which is both correct and not suspicious under both conventions.
The `json_ui.cc` file contained both data-parsing and UI-related
code. This commit moves the JSON parsing code to its own `json.cc`
file, to separate concerns, make compilation faster when changes are
made to either UI or parsing code, and make the parsing code more
accessible to fuzzers.
The signature of the following function:
```
auto parse_json(StringView json);
```
was changed to:
```
JsonResult parse_json(StringView json);
```
to avoid `auto` deduction issues at compile-time.
