Remove `activesupport` dependency

Removes the activesupport dependency.

The main reason for this is to fix evaluation in ruby 3.0.x, as seen in the test suites. We were regularly seeing the effects of https://bugs.ruby-lang.org/issues/17494, which caused unrecoverable hangs in the interpreter.

The workaround is to remove the activesupport dependency, which eliminates refined methods which trigger the problem.

Closes #3 (closed)

The one major change is the fact that we no longer permit concurrent class loading when evaluating class_for_class. Now that this is a library, and is expected to be loaded before any user code, this is likely to be less susceptible to deadlocks. But this is a risk, since I don't know much about the reasons why that code was added in the first place.