Replace grouped-parameter lists with new syntax for argument groups.

[toddjonker]

The new syntax (: ...) denotes an "argument group" and can only occur in an E-expression. It replaces the use of lists to pass multiple expressions to a single parameter.

"Grouped parameters" are removed from signatures, leaving only cardinality. To use multiple expressions with a parameter, the caller uses an argument group. A single expression (of appropriate shape) is always acceptable, regardless of cardinality.

In the template language, macro invocations can use the syntax (; ...) to denote argument groups.

As you can see from the text changes, this is more simple than the prior syntax and semantics. It turns grouping into a syntactic concern at the macro invocation site.

Issue #, if available:

This change is related to #291 but has at least one key difference. As proposed here, argument groups are not general-purpose streams, and can only appear in macro-argument positions. This is because they have a critical difference from the values macro: their contents are shape-constrained by the relevant macro parameter's type, most obviously for macro-shaped parameters. Personally, I'm in favor of keeping argument groups and values invocations syntactically distinct, to reduce confusion.

Other notes:

• Content explaining when groups are allowed needs to in sync with the binary encoding: text E-expressions should only allow groups where the binary form allows them.
• In editing this I discovered that these docs currently don't have description of text E-expressions; I'll have to circle back and get that content added.

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[toddjonker]

I believe we have consensus, modulo the concrete syntax. Holding off on merging to let people review my updates yesterday.

[toddjonker]

Ooops @popematt our messages passed in spacetime. Did you get a chance to see the latest updates?

[popematt]

@toddjonker Yes, I did see the updates.