Fix std/core crates (#2489)

- Add convert
- Add btree_set for without_std

substrate/core/sr-std/with_std.rs

@@ -32,6 +32,7 @@ pub use std::slice;
 pub use std::vec;
 pub use std::default;
 pub use std::result;
+pub use std::convert;
 
 pub mod collections {
 	pub use std::collections::btree_map;

substrate/core/sr-std/without_std.rs

@@ -64,10 +64,12 @@ pub use core::ptr;
 pub use core::slice;
 pub use core::default;
 pub use core::result;
+pub use core::convert;
 // We are trying to avoid certain things here, such as `core::string`
 // (if you need `String` you most probably doing something wrong, since
 // runtime doesn't require anything human readable).
 
 pub mod collections {
 	pub use alloc::collections::btree_map;
+	pub use alloc::collections::btree_set;
 }