fmt,cxxrtl: add UNICHAR format type.

This format type is used to print an Unicode character (code point) as
its UTF-8 serialization. To this end, two UTF-8 decoders (one for fmt,
one for cxxrtl) are added for rendering. When converted to a Verilog
format specifier, `UNICHAR` degrades to `%c` with the low 7 bits of
the code point, which has equivalent behavior for inputs not exceeding
ASCII. (SystemVerilog leaves source and display encodings completely
undefined.)

kernel/fmt.h

@@ -56,13 +56,14 @@ struct FmtPart {
 		LITERAL  	= 0,
 		INTEGER 	= 1,
 		STRING    = 2,
-		VLOG_TIME = 3,
+		UNICHAR   = 3,
+		VLOG_TIME = 4,
 	} type;
 
 	// LITERAL type
 	std::string str;
 
-	// INTEGER/STRING types
+	// INTEGER/STRING/UNICHAR types
 	RTLIL::SigSpec sig;
 
 	// INTEGER/STRING/VLOG_TIME types