Optimize the lexer

[Ana Gelez]

The current algorithm works well but is terribly slow. Another algorithm that should work as good and be much faster :

start = 0
end = 1
while start < src.len()
    matches = tokenize(src[start..end])
    if matches.len() == 1
        push matches[0]
        start = end
        end = start + 1
    else if matches.len() == 0
        error "unexpected character"
    else
        end = end + 1