Add s-expression parser to lib/util

[mkhan45]

I don't remember if you thought this should be added to util, but:

    def parser = SexpParser.new(filename, file_bytes);
    def res = parser.readSexp();
    match (res) {
        Success(res) => {
            ...
        }
        err => {
            ...
        }
    }

[titzer]

Because this is a parser that might be fed very large files, having position information for errors is really useful. Given that I think it makes sense that this parser take either an error generator object or has a mutable field for a callback on errors, that includes location information.

Then the parser can return a default value for the ADT (atom of an empty string) if it fails.

[mkhan45]

Does it make sense to just exploit the underlying TextReader's error fields?

    def readAtom() -> SExpr {
        skipWhitespace();
        def atom_end = star_rel(1, atomChar);
        if (atom_end == -1) {
	    fail("Expected atom");
	    return SExpr.Atom("");
	}

        def tok = readToken(atom_end - pos);
        skipWhitespace();
        return SExpr.Atom(tok.image);
    }

[titzer]

How about a list for the elements? Also, I think I'd prefer elems or elements as the field name.

[titzer]

Maybe SExpr as a name?