In preparation for the native semantics, we need more flexibility in
defining generic template instantiations for the Semantics classes. This
brings some minor incompatibilities for semantics implementations that
use partial specializations for things like Reference<T> and
EffectAxiom<T>.

some cleanup along the way

Implementations still TODO

not used or implemented currently

it's very readylog-specific and won't necessarily be useful for other
semantics impls

Same as the ExecutionContext, specific to readylog.

The Reference< > template gained a new parameter for the argument type.
It can be an arbitrary Expression or a Value. There is also a new,
specific event type for use with the exog queue, which makes use of
this new Reference<T, Value> (i.e. Grounding<T>).

This change may cause some minor breakage in downstream code, but it
should be straightforward to fix because the change is making compiler
errors more specific and adds some additional type safety around the
handling of events.

Want to stay compat with clang.

Preparation for moving list operations out of procedural.*