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.
