…ear#4440)

This PR introduces a strictly typed structure - `AccountId` that enforces account ID validation.
This way, we can ensure validity and thereby fail fast before an invalid account name is used.

This also, optionally, ensures validation on {,de}serialization so, yeah..

The structure is defined within its own self-contained crate `near-account-id` to allow for easy publishing.
The extra dependencies - `borsh`, `serde` are made optional to allow the crate to be lightweight enough to be depended upon.

Within `nearcore`, the structure is reexported via `near_primitives::AccountId` and the crate via `near_primitives{,_core}::account::id` with default features (`borsh`, `serde`) but you can opt-out of those by depending on it directly.

Resolves near#2074, supersedes near#2831.

* [x] Existing unit tests pass with minor updates
* [x] Nayduck tests pass without modifications
* [x] Manually ensured that the protocol is not changed