Keys cannot start with [.

This change breaks backward compatibility in theory, but supporting the
syntax `[key] = 1` is pretty annoying, and TOML should be easy to parse.

Concerns?
> Key/value pairs within tables are unordered.

For one, this seems to be tangential to this pull request's purpose.

What do you mean by `unordered` here? That order is not guaranteed? That they have not been sorted into some order?