cfunction 
    (json/encode  x  &opt  tab  newline  buf )
    Encodes  a  janet  value  in  JSON  (utf-8 ).  tab  and  newline  are  optional  
    byte  sequence  which  are  used  to  format  the  output  JSON.  if  buf  is  
    provided , the  formated  JSON  is  append  to  buf  instead  of  a  new  
    buffer.  Returns  the  modifed  buffer.