Asserts in release code

[tkcl81]

Hi, thanks for your work! There are some asserts which prevent proper error handling when using your crate. Could you please drop them or turn into debug_assert? Thanks!

            let slice = &src[slice_start_index..current_index];
            assert_eq!(slice, "}", "[Lexer] Invalid last char, should be '}}'");
            if slice != "}" {
                return Err(LexerError::InvalidLastChar);
            }

[d0rianb]

Fixed in #4