`foreach` expansions, value collections.

[BillWagner]

This is a followup to the issue transferred to dotnet/roslyn#84307

This does uncover questions on how we should describe foreach, and how we specify the behavior of identity conversions:

• Identity conversions and creating a copy: In v3, the language for an identity conversion did not prohibit copying the target. In v5 that language was added, seemingly to prohibit copying a struct. Initial analysis indicates this prohibition is correct. Is there any text on conversions that could be interpreted as allowing a copy for an identity conversion?
• Should the description of foreach make it clear that a value based collection could be mutated? Is that necessary in the normative language?
• Should we return to 2 expansions of foreach? One where the collection type has the necessary public members, and one where the collection type explicitly implements the expected interfaces?

There are a number of forward looking comments here: dotnet/roslyn#84307 (comment) When we reach the versions where ref struct types are allowed to implement some interfaces, we'll need to address some of them.

[jskeet]

We should check how using is defined in terms of an expansion as well; it may use an identity conversion expecting it not to cause a copy.

[jskeet]

@BillWagner will contact the Roslyn team to determine whether we really want an identity conversion to be a no-op (in which case we may want to change the spec to avoid being inconsistent with itself) or not.