Rename variables in Base64.* for readability (facebookincubator#11299)

Summary:
Follow-up PR: facebookincubator#10371

The following changes are done in this PR.
- Variable names are updated for better readability.
- Function declarations in Base64.h are reordered for better maintainability.

Pull Request resolved: facebookincubator#11299

Reviewed By: pedroerp

Differential Revision: D65850265

Pulled By: mbasmanova

fbshipit-source-id: 5b535a5c29238e373ac914f9cf027ac554414b41