Readonly structs in standard library

[KalleOlaviNiemitalo]

Describe the bug

Of the struct types listed in Annex C of the C# 7.x draft, only a few are specified as readonly struct. In .NET however, most of them are defined as readonly struct.
Is this allowed, and is it an extension?

Example

Types that are readonly struct in .NET but plain struct in Annex C include:

• System.Int32
• System.Span<T>
• System.Runtime.CompilerServices.TaskAwaiter<TResult>

Expected behavior

Could be solved in any of these ways:

• make them readonly struct in the standard, too
• declare that an implementation can add readonly to standard library struct types
• as above, but also change every readonly struct in Annex C to plain struct

Additional context

Similar to #870 on attributes.

[KalleOlaviNiemitalo]

See also #894 for an effect of readonly struct outside the struct type itself.

[jskeet]

Hmm... I'm tempted to go with the first option, although we'll probably need an explanatory note to explain why they're not the same as in ECMA 335.

@BillWagner, preferences for this? (I'm happy to create the PR.)

[BillWagner]

I like the first option with the Note: as well.

[jskeet]

Right - I'll try to prep a PR soon.