Proto/Token: rename "source" and "sink" to "giver" and "receiver"

Context

See the following discussion: !7373 (comment 1255111365).

Manually testing the MR

This is an internal change, it should not be visible from the outside. The Token module is well tested in integration/test_token.ml which can be launched using:

dune exec src/proto_alpha/lib_protocol/test/integration/main.exe -- test "^token"

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR