https://doc.rust-lang.org/beta/std/collections/btree_map/struct.IntoKeys.html
IntoKeys in std::collections::btree_map - Rust
An owning iterator over the keys of a `BTreeMap`.
stdcollectionsbtreemaprust
https://paritytech.github.io/polkadot-sdk/master/sp_std/collections/btree_map/struct.IntoKeys.html
IntoKeys in sp_std::collections::btree_map - Rust
An owning iterator over the keys of a `BTreeMap`.
spstdcollectionsbtreemap