Rollup merge of rust-lang#102556 - WaffleLapkin:implied_by_btree_new, r=Mark-Simulacrum

Make `feature(const_btree_len)` implied by `feature(const_btree_new)`

...this should fix code that used the old feature that was changed in rust-lang#102197

cc ```@davidtwco``` it seems like tidy doesn't check `implied_by`, should it?

Author: Dylan-DPC