fixed declarations

[tannergooding]

Summary

Much in the same way that using declarations have been enabled in the language, we should allow fixed declarations.

Motiviation

The motiviations, design, and considerations are essentially the same as for using declarations and fixed was listed as a possible future expansion in the original using declaration proposal: https://github.com/dotnet/csharplang/blob/master/proposals/csharp-8.0/using.md#future-expansions

[tannergooding]

CC. @jaredpar

I'm happy to actually fill in the details above, if that is more desirable.

[Perksey]

+1

[john-h-k]

would also love this much

[Sergio0694]

Definitely +1 to this 😄
Would simplify code a lot, especially when you have to do lots of short lived pinnings.

[nathanAjacobs]

This would be a great feature, hopefully it is implemented one day.

[IS4Code]

Still looking forward to this!