Migrate codebase to std::string_view (except z3++.h) (#8266)

* Initial plan

* Update z3 codebase to use std::string_view (except z3++.h)

- Updated params.cpp/h to use string_view internally for parameter descriptions
- Updated trace.h/cpp to accept string_view for trace tag functions
- Updated hash.h/cpp to use string_view for string_hash function
- Updated all callers of string_hash to use string_view
- Properly handled nullptr to empty string_view conversions
- All tests passing

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

* Add missing string_view includes to headers

Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: NikolajBjorner <3085284+NikolajBjorner@users.noreply.github.com>

src/ast/rewriter/cached_var_subst.h

@@ -29,7 +29,7 @@ class cached_var_subst {
     };
     struct key_hash_proc {
         unsigned operator()(key * k) const {
-            return string_hash(reinterpret_cast<char const *>(k->m_bindings), sizeof(expr *) * k->m_num_bindings, k->m_qa->get_id());
+            return string_hash(std::string_view(reinterpret_cast<char const *>(k->m_bindings), sizeof(expr *) * k->m_num_bindings), k->m_qa->get_id());
         }
     };
     struct key_eq_proc {

src/ast/sls/sls_seq_plugin.cpp

@@ -103,7 +103,7 @@ namespace sls {
     struct zstring_hash_proc {
         unsigned operator()(zstring const & s) const {
             auto str = s.encode();
-            return string_hash(str.c_str(), static_cast<unsigned>(s.length()), 17);
+            return string_hash(std::string_view(str), 17);
         }
     };