Variables that are collections of `CanonicalVarKind` are sometimes
called `var_kinds` and sometimes called `variables`. The former is much
better, because `variables` is (a) non-descript, and (b) often used
nearby for collections of `I::GenericArg`. I found the inconsistency
made the canonicalization code harder to understand.
This commit renames various `variables` things as `var_kinds`.
This commit desugars the drop and replace deriving from an
assignment at MIR build, avoiding the construction of the
DropAndReplace terminator (which will be removed in a followign PR)
In order to retain the same error messages for replaces a new
DesugaringKind::Replace variant is introduced.
Nicholas Nethercote