Disambiguate "value binding"

[geoffromer]

This proposal removes the definition of the term "value binding" as a primitive
category conversion from reference to value, replacing it with the term "value
acquisition". The other meaning of "value binding", a binding declared by a
value binding pattern, is unchanged.

[zygoloid]

Looks good to me; I'm going to run this past some safety folks and other leads to make sure we have alignment.

[chandlerc]

This proposal removes the definition of the term "value binding" as a primitive
category conversion from reference to value, replacing it with the term "value
borrowing".

I'm not opposed to trying to find some distinct terminology here, but I don't particularly like "value borrowing".

Specifically, I'm very hesitant to begin using the term "borrow" until we have our safety design in mind and can make sure it a) doesn't conflict with that terminology, and b) isn't confusing when compared with Rust's use of the term.

While maybe we end up with these looking very much like a "shared borrow" in Rust, jumping to that kind of terminology now makes me very nervous.

It also seems hard to square with the existence of copy value representations.

[geoffromer]

I've renamed "value borrowing" to "value acquisition", per Discord/offline discussion.

[zygoloid]

Approving for leads, thanks!