make overlong Chars !isvalid; throw error on UInt32 of overlong Char

[JeffBezanson]

This takes care of most of #25452.

• Handle an extra case of overlong encoding.
• Make isvalid(::Char) return false on overlongs.
• Make UInt32(::Char) throw an error on overlongs.

[nalimilan]

CI failures look legitimate.

[StefanKarpinski]

I think this pushes the decoding function checks to the point where bit-twiddling will be less effective than a set of range checks. I can take a crack at that version and see how it goes. I'm not sure about introducing this isoverlong predicate. Why not just have isvalid? I guess it's somewhat informative.

[StefanKarpinski]

Another thing: since we're going to throw for all invalid characters, we may as well replace MalformedChar with InvalidChar and just lump all forms of invalidness together. The point of the previous distinction that since we didn't throw for all invalid characters, calling it InvalidChar would be misleading – it was only truly malformed byte sequences that caused an error before.

[JeffBezanson]

Updated.

I think isoverlong is fine for now; it's not exported (wasn't introduced by this PR either).

[StefanKarpinski]

Looks good to me. We can make performance tweaks in any time – this is the behavior we want.