Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/data/Compressor.cpp

@@ -379,6 +379,11 @@ bool Compressor::getConstant(Format in, const char *&out)
 	return formatNames.find(in, out);
 }
 
+std::vector<std::string> Compressor::getConstants(Format)
+{
+	return formatNames.getNames();
+}
+
 StringMap<Compressor::Format, Compressor::FORMAT_MAX_ENUM>::Entry Compressor::formatEntries[] =
 {
 	{ "lz4",  FORMAT_LZ4  },