cxxrtl: refactor the formatter and use a closure.

This commit achieves three roughly equally important goals:
1. To bring the rendering code in kernel/fmt.cc and in cxxrtl.h as close
   together as possible, with an ideal of only having the bigint library
   as the difference between the render functions.
2. To make the treatment of `$time` and `$realtime` in CXXRTL closer to
   the Verilog semantics, at least in the formatting code.
3. To change the code generator so that all of the `$print`-to-`string`
   conversion code is contained inside of a closure.

There are two reasons to aim for goal (3):
a. Because output redirection through definition of a global ostream
   object is neither convenient nor useful for environments where
   the output is consumed by other code rather than being printed on
   a terminal.
b. Because it may be desirable to, in some cases, ignore the `$print`
   cells that are present in the netlist based on a runtime decision.
   This is doubly true for an upcoming `$check` cell implementing
   assertions, since failing a `$check` would by default cause a crash.

backends/cxxrtl/cxxrtl_backend.cc

@@ -1072,9 +1072,12 @@ struct CxxrtlWorker {
 		dump_sigspec_rhs(cell->getPort(ID::EN));
 		f << " == value<1>{1u}) {\n";
 		inc_indent();
-			f << indent << print_output;
-			fmt.emit_cxxrtl(f, [this](const RTLIL::SigSpec &sig) { dump_sigspec_rhs(sig); });
-			f << ";\n";
+			f << indent << "auto formatter = [&](int64_t itime, double ftime) {\n";
+			inc_indent();
+				fmt.emit_cxxrtl(f, indent, [this](const RTLIL::SigSpec &sig) { dump_sigspec_rhs(sig); });
+			dec_indent();
+			f << indent << "};\n";
+			f << indent << print_output << " << formatter(steps, steps);\n";
 		dec_indent();
 		f << indent << "}\n";
 	}