Add Design::run_pass() API for programmatic pass execution

This commit adds a new run_pass() method to the RTLIL::Design class,
providing a convenient API for executing Yosys passes programmatically.

This is particularly useful for PyYosys users who want to run passes
on a design object without needing to manually construct Pass::call()
invocations. The method wraps Pass::call() with appropriate logging
to maintain consistency with command-line pass execution.

Example usage (from Python):
    design = ys.Design()
    # ... build or load design ...
    design.run_pass("hierarchy")
    design.run_pass("proc")
    design.run_pass("opt")

Changes:
- kernel/rtlil.h: Add run_pass() method declaration
- kernel/rtlil.cc: Implement run_pass() method
- tests/unit/kernel/test_design_run_pass.cc: Add unit tests

kernel/rtlil.cc

@@ -1610,6 +1610,13 @@ std::vector<RTLIL::Module*> RTLIL::Design::selected_modules(RTLIL::SelectPartial
 	return result;
 }
 
+void RTLIL::Design::run_pass(std::string command)
+{
+	log("\n-- Running command `%s' --\n", command.c_str());
+	Pass::call(this, command);
+	log_flush();
+}
+
 RTLIL::Module::Module()
 {
 	static unsigned int hashidx_count = 123456789;