Remove 'external' functions

[nventuro]

The goal behind making every function overrideable (virtual) is to, well, allow for overrides. Often these are 'extensions' of the original function, adding new checks, events, side effects, etc., and therefore require calling into the original:

function foo() override {
  super.foo();
  do_more_things();
}

However, this is not possible if foo is external, because super.foo() is considered an internal function call by the language (!).

This PR changes all external functions and makes them public. This creates a slight issue where external and internal functions had similar names: they will now both be internally-callable, which may lead to hard to detect mistakes. A prime example were AccessControl's grantRole and _grantRole. After some consideration, we decided to avoid these scenarios, by either renaming the internal functions, or removing them where we considered they weren't really necessary.

This PR is best reviewed commit-by-commit.