Add support for marshalling JSON

[GoogleCodeExporter]

Could vim have a built-in tojson({value}) and fromjson({json}) helpers to 
serialize and deserialize JSON?

JSON is used in lots of vim plugins like 
https://github.com/Valloric/YouCompleteMe, 
https://github.com/google/vim-maktaba, 
https://github.com/MarcWeber/vim-addon-manager, and eventually Vundle 
(https://github.com/VundleVim/Vundle.vim/pull/560). These can either use slow 
hacks or depend on python support, but it would be best if vim just had native, 
performant support for JSON marshalling built in.

Expected behavior
  :echo tojson({'a': [1, 'foo'], 'b': 2.1}) ==# '{"a": [1, "foo"], "b": 2.1}'
  1
  :echo fromjson("[1.0, {}, []]") ==# [1.0, {}, []]
  1
  :echo tojson(fromjson('[null, true, false]')) ==# '[null, true, false]'
  1

Note in the last example there needs to be a way to represent null, true, and 
false unambiguously even though vim doesn't have these primitives. Also, 
fromjson() could use an option to translate into standard vim equivalents like 
'', 1, and 0.

Original issue reported on code.google.com by [email protected] on 13 Jul 2015 at 11:30