Rename "Value expressions and place expressions" section to "Expression categories"

[Havvy]

C++ calls them value categories. Since we call one "value expression", I'd like to avoid that term and instead call them a category of some kind. Since the distinction is about what kind of thing the expression evaluates to, I figured "evaluation category" would be the best term.

Having this term would also let me say parenthesized expressions are of the same evaluation category as their enclosed operand.

[Havvy]

Nominating with the question that adding this term is correct and that the name is alright.

[nikomatsakis]

Perhaps "categories of expressions" or "expression categories"?

[nikomatsakis]

Evaluation seems strange to me because it sounds very much like a property of what happens when the expression executes, but this is more of a category of expression that can be evaluated to a place in memory in some contexts.

[joshtriplett]

Agreed. We're talking about a name that encompasses "value expression" and "place expression", so "expression category" seems reasonable.