More `const` with MSRV 1.83

[tomcur]

Once Color gets an MSRV of 1.83, some more methods can be marked const:

• Missing::insert
• Flags::set_missing
• Flags::set_named_color_space
• Flags::discard_name

[DJMcNab]

I wouldn't necessarily be opposed to doing that MSRV bump now. OTOH, these are relatively minor wins that it's probably not worth it?

[tomcur]

I don't think they're so important that it needs to land now, but I did want to track it so we remember to do this later. It can make some of the flag/missing manipulations in const contexts a bit more ergonomic.

That said, I'd also be fine with doing the bump now.

[DJMcNab]

My suggestion would be to add warn(clippy::missing_const_for_fn) on the specific items, so that when the MSRV bump does happen, we get a direct warning (I'd consider even enabling that lint globally, so that we are forced to make an explicit decision...)

[waywardmonkeys]

I am already delaying trying to introduce color to some things due to MSRV, so I am happy to track these things, but less excited about them actually landing yet.