Add readonly modifier to standard library structs.

[jskeet]

The remaining non-readonly structs are:

• Nullable<T> (not sure why)
• ValueTuple<...> (as it's mutable)

Fixes #893.

@Nigel-Ecma and @RexJaeschke I'd particularly like your feedback on the note. Perhaps it shouldn't even be a note, but normative text?

[BillWagner]

This LGTM.

[Nigel-Ecma]

Change “standard” to ”specification” as per @KalleOlaviNiemitalo (I don’t recall this change myself doing the diff for #900 correctness, it showed up a lot, I'm a “Standard” person myself)

Otherwise LGTM and I pre-approve.

[jskeet]

Changed standard to specification. I'll "dismiss" Nigel's review given the pre-approval.

Pre-approved as I've made the requested change.

[KalleOlaviNiemitalo]

• Nullable<T> (not sure why)

That was explained in dotnet/corefx#24997 (comment): the Equals(object), GetHashCode(), and ToString() methods of Nullable<T> call the corresponding methods of T, and those may mutate the T instance stored within the Nullable<T>. If Nullable<T> were changed to a readonly struct, then its methods would make defensive copies of T and the mutations would no longer affect the Nullable<T> instance. That difference could break some applications.