Allow adding custom ToKey implementation

[vasyafromrussia]

In our Fungible Token smart contract we need to resolve LookupMap keys in a special way. It made us fork near-sdk-rs to add the LookupMapAdapter. The necessity of this decision is due to the fact that ToKey implementations are sealed here, and we cannot instantiate LookupMap using with_hasher.

So I wonder what's a reason behind sealing these implementations, and probably it's worth to get rid of this sealing and allow custom implementations?