feat: card id

[reez]

Resolves #50

Description

Expose the card id, and expose it thru swift bindings as well.

Notes to the reviewers

Added data-encoding for base32, I didn't see a std equivalent, let me know if there is a different/better way if there is one.

https://github.com/coinkite/coinkite-tap-proto/blob/master/docs/protocol.md#pubkeys

Both types of card have a unique pubkey, mapped into a human-readable hash with this process:

SHA-256 (compressed pubkey = 33 bytes)
Skip the first 8 bytes (because that's revealed in the NFC URL)
Encode as base32 and take the first 20 characters in four groups of five characters
Insert dashes between the groups.
The result is 23 characters long.

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[notmandatory]

ACK da263ab

Nice job, looks like you're getting the hang of rust coding!