Add a test suite for json.reader. Fix bugs in json.reader: failure to parse when...

Add a test suite for json.reader. Fix bugs in json.reader: failure to parse when more than one character of whitespace between tokens, failure to parse escape codes in strings, misinterpretation of "e+02" as a negative exponent, failure to handle leading whitespace, and failure to handle "+" as the first character of a number. Change json.reader to parse JSON booleans into Factor t and f, and to parse JSON null into the json-null symbol.