librustc: Eliminate the `ref` syntax for unboxed closure capture clauses
in favor of `move`.

This breaks code that used `move` as an identifier, because it is now a
keyword. Change such identifiers to not use the keyword `move`.
Additionally, this breaks code that was counting on by-value or
by-reference capture semantics for unboxed closures (behind the feature
gate). Change `ref |:|` to `|:|` and `|:|` to `move |:|`.

Part of RFC #63; part of issue #12831.

[breaking-change]
auto merge of #17519 : pcwalton/rust/unboxed-closure-move-syntax, r=nikomatsakis

in favor of `move`.

This breaks code that used `move` as an identifier, because it is now a
keyword. Change such identifiers to not use the keyword `move`.
Additionally, this breaks code that was counting on by-value or
by-reference capture semantics for unboxed closures (behind the feature
gate). Change `ref |:|` to `|:|` and `|:|` to `move |:|`.

Part of RFC #63; part of issue #12831.

[breaking-change]

r? @nikomatsakis