Reconsider Struct and Enum Variant Expressions

[Havvy]

Although the semantic of what is actually created differs based on the path, they have the same syntax and the struct expression page also includes unions. They were separate expressions when all expressions were listed on one page, but next to each other. It was easy to understand that way. But that's not been the case for a long time now. If my assessment is correct, then these pages should be merged together. So, is my assessment correct?

[Centril]

It makes sense to me to treat these both as a sort of "product type expression", but I'd check with a 2nd person to make sure.

[Havvy]

@Centril, @scottmcm, and I discussed this further on Discord. We ultimately came to a conclusion of the following action items:

1. Merge enum struct variant literals and struct literals into a "Braced Literal Expression"
2. Move tuple struct and enum tuple variants into the "Call Expression"

We considered renaming "Struct expression" to "Nominal product type expression" and noted that tuple structs are really function calls when constructing.

[ehuss]

Seems reasonable to me, since they are essentially identical. It will just need to retain the note somewhere that functional record update syntax isn't supported (which seems like a weird limitation to me).

I think the grammar rule names should be renamed, I don't like what I picked.