FSet 2.0 update

Someone asked me what the rationale is for the decision, in my FSet 2.0 release candidate, to have no default default for maps and seqs, so that an out-of-domain lookup will signal an error.  I started to write an answer, but after putting the arguments for and against this change down on the page and mulling them over for a few days, I concluded it was a mistake and decided to reverse it.

So in FSet 2.0, it will still be the case, unless you specify otherwise, that an out-of-domain lookup on a map, or an out-of-bounds lookup on a seq, will simply return nil (with a nil second value).  You do, as before, have the option to specify a different default, and now you also have the option to specify no default, if you want out-of-domain/bounds lookups to signal an error.

I have tagged v2.0.0-rc1.