feat(stdlib): encode and decode base16

[zouguangxian]

Motivation

provide a human-friendly representation of binary-coded values.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

move test passed in language/move-stdlib/nursery/sources directory.

[sblackshear]

I am personally in favor of having more of these utility encoding/decoding libraries inside the Move stdlib, but we need to think carefully about how we want to organize them--one module per format (as this suggests?). All in one big bytes module? Something else?

[jolestar]

I support one module per format

[sblackshear]

Another approach we could consider here (that would make the testing burden a bit less onerous) is to implement this via native functions that wrap the Rust hex library.

[jolestar]

Another approach we could consider here (that would make the testing burden a bit less onerous) is to implement this via native functions that wrap the Rust hex library.

I also think it is possible to let the User Module implement a native function with a WASM runtime in the future. This approach lets adding new native functions more gracefully and does not need a hard fork upgrade.