feat: CRP-2757 implement Motoko backend

[altkdf]

Implements:

• The actual backend.
• Tests that don't require VetKD.
• Github workflow.

[altkdf]

The map type operations currently incur much more overhead than they should and the reason is that the map types in motoko don't yet support range functions that we need for efficiency. As it looks, this limitation should soon be removed though caffeinelabs/motoko-core#271.

[altkdf]

In next PRs: motoko-based canisters, better documentation, pocket IC tests checking that the VetKD calls work as intended.

[fspreiss]

Thanks, @altkdf! Looks generally good! Please find a few comments below.

[fspreiss]

Thanks for addressing the comments and for creating the tickets for the remaining ones, @altkdf!