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?
Sorry, `diff` does not work well when the positions of line breaks are changed, and I'm not forgoing 80 columns. :-) (That stuff was already there.)

N.B. But yes, unordered means that key/value pairs are not guaranteed to appear in any particular order. AFAIK, this is a common property of maps.