Remove redundant default constructors when they're the only constructor (#8461)

* Initial plan

* Modernize C++ constructors to use C++11 default member initialization - Phase 1

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Fix theory_pb.h struct definition - move reset() back inside struct

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Modernize C++ constructors to use C++11 default member initialization - Phase 2

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Fix opt_solver.h - revert rational initialization (complex type)

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Modernize C++ constructors to use C++11 default member initialization - Phase 3

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Fix sparse_matrix.h - explicitly initialize union member in default constructor

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

* Remove unnecessary default constructors when they're the only constructor

Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: nunoplopes <2998477+nunoplopes@users.noreply.github.com>

src/sat/sat_lookahead.h

@@ -116,15 +116,13 @@ namespace sat {
         };
 
         struct prefix {
-            unsigned m_prefix;
-            unsigned m_length;
-            prefix(): m_prefix(0), m_length(0) {}            
+            unsigned m_prefix = 0;
+            unsigned m_length = 0;
         };
 
         struct lit_info {
-            double     m_lookahead_reward;
-            unsigned  m_double_lookahead;
-            lit_info(): m_lookahead_reward(0), m_double_lookahead(0) {}
+            double     m_lookahead_reward = 0;
+            unsigned  m_double_lookahead = 0;
         };
 
         struct stats {

src/sat/smt/euf_ackerman.h

@@ -29,10 +29,10 @@ namespace euf {
     class ackerman {
 
         struct inference : dll_base<inference>{
-            expr* a, *b, *c;
+            expr* a = nullptr, *b = nullptr, *c = nullptr;
             unsigned   m_count{ 0 };
-            bool is_cc;
-            inference(): a(nullptr), b(nullptr), c(nullptr), is_cc(false) {}
+            bool is_cc = false;
+            inference() = default;
             inference(app* a, app* b): a(a), b(b), c(nullptr), is_cc(true) {}
             inference(expr* a, expr* b, expr* c): a(a), b(b), c(c), is_cc(false) {}
         };

src/sat/smt/sat_smt.h

@@ -30,14 +30,13 @@ namespace sat {
     };
 
     class constraint_base {
-        extension* m_ex;
+        extension* m_ex = nullptr;
         unsigned   m_mem[0];
         static size_t ext_size() {
             return sizeof(((constraint_base*)nullptr)->m_ex);
         }
 
     public:
-        constraint_base(): m_ex(nullptr) {}
         void*  mem() { return m_mem; }
 
         static size_t obj_size(size_t sz) {