Add BoundedBTreeMap to frame_support::storage (#8745)

* Add `BoundedBTreeMap` to `frame_support::storage`

Part of https://github.com/paritytech/substrate/issues/8719.

* max_encoded_len will never encode length > bound

* requiring users to maintain an unchecked invariant is unsafe

* only impl debug when std

* add some marker traits

* add tests

substrate/frame/support/src/storage/mod.rs

@@ -29,6 +29,7 @@ pub use sp_runtime::TransactionOutcome;
 
 pub mod unhashed;
 pub mod hashed;
+pub mod bounded_btree_map;
 pub mod bounded_vec;
 pub mod child;
 #[doc(hidden)]
@@ -817,6 +818,7 @@ mod private {
 	impl<T: Encode> Sealed for Vec<T> {}
 	impl<Hash: Encode> Sealed for Digest<Hash> {}
 	impl<T: BoundedVecValue, S: Get<u32>> Sealed for BoundedVec<T, S> {}
+	impl<K, V, S> Sealed for bounded_btree_map::BoundedBTreeMap<K, V, S> {}
 }
 
 impl<T: Encode> StorageAppend<T> for Vec<T> {}