C# 7.x: Add support for readonly structs

[RexJaeschke]

This PR adds one new modifier to structs, readonly, in struct_modifier.

[KalleOlaviNiemitalo]

Should an implicit conversion from dynamic to a ref struct type be rejected at compile time? Microsoft's implementation of dynamic uses the ref struct as a type argument, which is then not supported by the CLR. However, as the C# standard does not specify the details of dynamic, I imagine a different compiler with a different library could find a way to make the conversion work. dotnet/csharplang#4713, #282

[gafter]

This draft does not reflect the draft specification at https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.2/span-safety.md

[RexJaeschke]

Reminder to look at #334 for other open issues w.r.t this PR (including Neal's concerns re missing text re span safety).

[RexJaeschke]

Based on Neal’s suggestion during yesterday's TG2 meeting, we agreed to merge as is, but only call readonly structs completed. Then, create another PR for all the additional work for ref structs.

[BillWagner]

I'll rebase this branch to resolve conflicts, and mark it ready for review.

[jskeet]

Assigned to @gafter for the splitting off - we should have assigned this before...

[gafter]

I've removed ref struct from this PR and moved it to #601. This should be fine for readonly struct.

[BillWagner]

@jskeet

We should be able to do the final approval for this and merge it during this week's meeting. The ref struct portion has all been moved to #601

[BillWagner]

This LGTM. Let's

[jskeet]

Yup, just done a final review. Will merge now.