First set of changes for tuples from feature spec and LDM notes.

[BillWagner]

Fixes dotnet/docs#15046

This PR updates the C# 7.0 proposal document so that it could be used as a proposed delta to the C# standard.

To make these edits, I did the following:

1. Update the language to conform to the standard language.
2. Update the changes based on C# LDM meeting notes in this repo, and the dotnet/roslyn repo. Note that I only incorporated changes that were released in C# 7.0. Later changes to tuples (such as equality) are not included.
3. Edit and review.
4. Test against current compiler set to <LangVersion>7.0</LangVersion>.

This is the first PR for this larger task. I have notes for reviewers, and questions about how to continue proceeding.

Questions / notes for reviewers:

1. Are the proposals useful in their current form? This PR updates the tuples proposal in place, assuming the proposal has served its purpose.
2. Are the notes about where changes / additions to the standard go clear? If not, does anyone have a better formatting suggestion?
3. This PR removes language about how the IDE would treat tuples, or made them informative note text. Is that the best strategy?
4. I removed any text that communicated a decision to not implement a behavior. Are there cases where we should do that?
5. I'm not sure the content on overloading / overriding and natural type is in the correct location. Should it move to be in "overload resolution" in general?

Thoughts on proceeding with other proposals:

This will help keep the C# language reference section of docs concise. I will keep making progress on these (although slowly). After finishing each release, I think a second set of edits should re-organize the changes to the standard by major clause rather than feature. (Updates to Lexical structure(, updates to Basic concepts, Types, Variables, etc). Thoughts on that work?

/cc @MadsTorgersen @gafter @jcouv @agocke @jaredpar

[BillWagner]

I would like at least one approval before this gets merged. It's larger, and has more semantic changes than I feel comfortable merging without thorough review.

[BillWagner]

I changed the base branch to a feature branch that @RexJaeschke and I will use to keep working on this project.

I'll merge this PR into that branch. Then, Rex and I will keep iterating on this (and feature specs).

[gafter]

I am disappointed that this was merged without addressing the many comments. The comments are not even retained as issues somewhere, so I'm not sure how you intend to track them.

[BillWagner]

@gafter I'll be adding issues for all those comments by tomorrow morning. They'll be in this project: https://github.com/dotnet/csharplang/projects/5

As I add the issues, I'll add links to all the original comments so we don't lose any of the review work.