Add more existentials documentation

[ldct]

This PR contains a few changes to add more documentation around existentials. Note that the "Protocols" chapter comes before the "OpaqueTypes" chapter.

• Expand the section Protocols as Types with other examples of using protocols as types
  • Personally I think these are a more common than "as a generic constraint", and hence deserve some elaboration
• Explicitly call the result of protocol composition a Protocol Composition Type.
  • Prior to this PR, the result does not have an explicit name in the Language Guide. Protocol Composition Type is the named used by the Language Reference.
  • The Generics chapter implicitly uses the name Protocol Composition for this. In my mind, this is a short form.
• Explicitly say that existentials can use protocol composition types.
• Delete the "opaque type" section in the protocols chapter. The same material seems to be present in the OpaqueTypes chapter.

[ldct]

ncc1701 is defined on line 233

[ldct]

this protocol is also defined in line 1629 of the "Protocols" chapter

[amartini51]

Thanks for writing this up as a pull request, and in particular thanks for taking the time to include the cross references between running code listings. I'll be out of the office next week for the Thanksgiving holiday, and will continue working with you on these changes when I get back. A few initial notes:

• It would be helpful to understand more detail about the problem in TSPL you’re trying to solve, and why this is the best approach. It looks like you’re adding information about the any P & Q syntax — for example, why a new section, rather than adding to the existing sections?
• The text that was deleted should be kept; it contrasts opaque and boxed types as part of introducing boxed types, with the aim of not requiring the reader to jump forward to the upcoming chapter.
• We’ll need to do some cleanup for style. For example, TSPL specifically avoids the term “existential”, and uses the full words for “enumerations” and “structures” rather than the jargon terms “enum” and “struct”. These are usually pretty easy for me to clean up, but you can also look at the TSPL style guide for more information.

[ldct]

Thanks for the review!

It looks like you’re adding information about the any P & Q syntax

Yes, I was trying to look up what the existentials any syntax supports (i.e. exactly what is legal to put in let x = any ???) and I saw that the section on Boxed Protocol Types is wrong (it says "any before the name of a protocol", which excludes P & Q unless you define "name of a protocol" in a rather unusual (to my mind) way)

The text that was deleted should be kept; it contrasts opaque and boxed types as part of introducing boxed types, with the aim of not requiring the reader to jump forward to the upcoming chapter.

Sure, I can revert this, but it seems strange to me that the Protocols chapter (which comes before the Generics chapter) talks about opaque types.

[ldct]

The language reference calls this a "protocol composition type", but I think the compiler warnings call this a "protocol-constrained type" 🤔