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/graphics/vertex.cpp

@@ -254,6 +254,11 @@ bool getConstant(IndexDataType in, const char *&out)
 	return indexTypes.find(in, out);
 }
 
+std::vector<std::string> getConstants(IndexDataType)
+{
+	return indexTypes.getNames();
+}
+
 bool getConstant(const char *in, Usage &out)
 {
 	return usages.find(in, out);
@@ -264,6 +269,11 @@ bool getConstant(Usage in, const char *&out)
 	return usages.find(in, out);
 }
 
+std::vector<std::string> getConstants(Usage)
+{
+	return usages.getNames();
+}
+
 } // vertex
 } // graphics
 } // love