Rollup merge of #144292 - joshtriplett:mbe-use-concrete-type-for-get-unused-rule, r=petrochenkov

mbe: Use concrete type for `get_unused_rule`

Rather than adding `get_unused_rule` to the `TTMacroExpander` trait, put
it on the concrete `MacroRulesMacroExpander`, and downcast to that type
via `Any` in order to call it.

Suggested-by: Vadim Petrochenkov <[email protected]>

r? ```````@petrochenkov```````

Author: matthiaskrgr