add encode_/decode_json() functions

The JSON docs refer to to_/from_ as being suboptimal given that they don't
handle utf8 properly.  (apparently.)  This change adds two additional autobox
methods using the preferred functions, while leaving the existing ones intact,
for anyone using them already.
Merge pull request #1 from RsrchBoy/topic/utf8

add encode_/decode_json() functions