Implement Value::demand()

[res2k]

There's a declaration of a Value::demand() method, but an implementation is nowhere to be found.

It would have some utility, though: it's the only method to create a named key that takes [begin, end) arguments. This is useful if you want to use a key stored in a string_view.

Note that I also changed the return value of demand():

• I think a non-const pointer is more useful as you can immediately modify the Value.
• Leaving it as a pointer keeps some symmetry with find().
• This change doesn't affect compatibility much, as demand() wasn't implemented, thus there can't be any user of it.

[cdunn2001]

Seems fine. I don't remember why this was not implemented. Personally, I like demand(), so thanks for this.

But you need to bump the minor version number.

[res2k]

But you need to bump the minor version number.

I guess the SOVERSION as well?

[cdunn2001]

Yeah, that needs to match.