Subterms Theory (#8115)

* somewhaat failed attempt at declaring subterm predicate

I can't really figure out how to link the smt parser to the rest of the
machinenery, so I will stop here and try from the other side. I'll start
implmenting the logic and see if it brings me back to the parser.

* initial logic implmentation

Very primitive, but I don't like have that much work uncommitted.

* parser implementation

* more theory

* Working base

* subterm reflexivity

* a few optimization

Skip adding obvious equalities or disequality

* removed some optimisations

* better handling of backtracking

* stupid segfault

Add m_subterm to the trail

* Update src/smt/theory_datatype.h

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/ast/rewriter/datatype_rewriter.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/smt/theory_datatype.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/smt/theory_datatype.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update src/smt/theory_datatype.cpp

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* review

* forgot to update `iterate_subterm`'s signature

* fix iterator segfault

* Remove duplicate include statement

Removed duplicate include of 'theory_datatype.h'.

* Replace 'optional' with 'std::option' in datatype_decl_plugin.h

* Add is_subterm_predicate matcher to datatype_decl_plugin

* Change std::option to std::optional for m_subterm

* Update pdecl.h

* Change has_subterm to use has_value method

* Update pdecl.cpp

---------

Co-authored-by: Nikolaj Bjorner <nbjorner@microsoft.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

src/ast/rewriter/datatype_rewriter.cpp

@@ -121,6 +121,9 @@ br_status datatype_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr
         result = m().mk_app(c_decl, num, new_args.data());
         return BR_DONE;        
     }
+    case OP_DT_SUBTERM:
+        // No rewrite yet for subterms
+        return BR_FAILED;
     default:
         UNREACHABLE();
     }