Remove quoting from bear_json_escape_string

It doesn't belong there. Instead, add numeric escaping and more symbolic
escapes, because why not.
Genericise bear_json_escape_strings to bear_strings_transform

It'll become handy later.
Rename json.[ch] to stringtransform.[ch]

Rename the function to put "string" first, too. Another transformation
will appear shortly.
Simplify test_json: use 2 variables instead of 3
Add shell escaping/quoting