Separate custom matched grounded atoms and grounded atoms matched by equality

[vsbogd]

Separate custom matched grounded atoms and grounded atoms matched by equality to address https://github.com/trueagi-io/hyperon-experimental/pull/698/files#r1623809472

We cannot understand whether custom matching is implemented on a grounded atom because of lack of the specialization feature in stable Rust, so I compared two approaches:

• returning optional matching procedure fn Grounded::get_match(&self) -> Option<Box<dyn Fn(&Atom) -> MatchResultsIter>>
• and adding separate fn Grounded::as_match(&self) -> Option<&dyn CustomMatch>
• having separate flag Grounded::has_custom_matching

I chosen last one because:

• it seems more clean and logical to me having each separate customization aspect as a separate trait
• if specialization is available in a stable Rust we can automatize it
• returning closures from get_match requires additional boxing and looks more surprising than implementing a trait

I like this approach finally and I think we could reuse it to implement Hash on value-like grounded atoms and to move execute into a separate trait.

Despite change is big most interesting files are:

• lib/src/atom/mod.rs
• lib/src/common/shared.rs
• c/src/atom.rs

Other files contains removed default match_by_equality logic and rearranging custom matching logic to match new API.

@Adam-Vandervorst FYI (cannot add you as a reviewer).