Before, only the syntax level (parse_*) was able to set the error
string. This patch fixes the situation so that lexical (lex_*) and
stream (stream_*) levels can report detailed error messages.
Also, instead of 0, EOF is now returned by stream on error.
It's no longer needed to load the whole input into a string and then
parse from the string. Instead, the input is read as needed from
a string or file.
Before, json_loads checked for '[' or '{' at the beginning. Now
there's a dedicated function for that: parse_json(). Also rename
parse() to parse_value().
Nothing was appended to strbuffer, so the buffer was left empty. An
empty strbuffer is not an empty string but NULL, so the result was a
segfault.
This patch fixes the problem by initializing strbuffer to an empty
string.
