Binding the root of a ^:once Var should emit a warning

[arrdem]

Not an error, because occasionally people do want to refresh ^:once defs. Maybe this is a redundant warning because a warning that ^:once is being discarded will already be emitted. Also the question of whether this should be implemented as part of the compiler in that a def form should emit this check, or whether it should be implemented as part of Var so that any alterRoot/bindRoot/swapRoot/commuteRoot call will emit such a warning.