add ContainsProxied template

[WebFreak001]

works like Contains, but in case there is a Proxy!T, it is also matched for T.

I originally made https://github.com/libmir/mir-ion/pull/22 but while that was on there I redid the changes again and this came up as useful.

I would be opening a PR in mir-ion that uses this to support e.g. Variant!(int, ProxiedType) to allow possible implicit conversions, serdeProxyCast conversions and regular to!T conversion from deserialized type to proxy type through the variant.