System.Collections.Immutable and APTCA/SecurityTransparentAttribute

[mkosieradzki]

If possible please add one of attributes forcing assembly to be Security Transparent (AllowPartiallyTrustedCallersAttribute or SecurityTransparentAttribute).

This will help using this library in some quite uncommon partial trust scenarios.

[joshfree]

/cc @morganbr

[morganbr]

As I mentioned in dotnet/corefx#3293, making it transparent sounds reasonable to me.