vpslot_engine to optional/mandatory

optional/mandatory have better support for extended JSON features.

It looks like all the vpslots would be better served as having an
"slot": { ... } field, instead of the SLOT flag enabling more fields
in the base part. extend/delete could provide the necessary extension
features.

But that's a much larger overhaul.

The was_loaded per slot is necessary for it to correctly initialize it -
it cannot use the same as the vpart_info, because even if a vpart_info
was loaded, the engine slot may not have been.

Author: ehughsbaird

src/veh_type.cpp

@@ -363,18 +363,12 @@ void vpart_info::load( const JsonObject &jo, const std::string &src )
         damage_reduction = load_damage_map( dred );
     }
 
+    // TODO?: move this into an object?
     if( has_flag( "ENGINE" ) ) {
         if( !engine_info ) {
             engine_info.emplace();
         }
-        assign( jo, "backfire_threshold", engine_info->backfire_threshold, strict );
-        assign( jo, "backfire_freq", engine_info->backfire_freq, strict );
-        assign( jo, "noise_factor", engine_info->noise_factor, strict );
-        assign( jo, "damaged_power_factor", engine_info->damaged_power_factor, strict );
-        assign( jo, "m2c", engine_info->m2c, strict );
-        assign( jo, "muscle_power_factor", engine_info->muscle_power_factor, strict );
-        assign( jo, "exclusions", engine_info->exclusions, strict );
-        assign( jo, "fuel_options", engine_info->fuel_opts, strict );
+        engine_info->deserialize( jo );
     }
 
     if( has_flag( "WHEEL" ) ) {
@@ -437,6 +431,20 @@ void vpart_info::load( const JsonObject &jo, const std::string &src )
     }
 }
 
+void vpslot_engine::deserialize( const JsonObject &jo )
+{
+    optional( jo, was_loaded, "backfire_threshold", backfire_threshold, 0.f );
+    optional( jo, was_loaded, "backfire_freq", backfire_freq, 1 );
+    optional( jo, was_loaded, "noise_factor", noise_factor, 0 );
+    optional( jo, was_loaded, "damaged_power_factor", damaged_power_factor, 0.f );
+    optional( jo, was_loaded, "m2c", m2c, 100 );
+    optional( jo, was_loaded, "muscle_power_factor", muscle_power_factor, 0 );
+    optional( jo, was_loaded, "exclusions", exclusions, string_reader{} );
+    optional( jo, was_loaded, "fuel_options", fuel_opts, string_id_reader<itype> {} );
+
+    was_loaded = true;
+}
+
 void vpart_info::set_flag( const std::string &flag )
 {
     flags.insert( flag );

src/veh_type.h

@@ -122,6 +122,8 @@ enum vpart_bitflags : int {
 };
 
 struct vpslot_engine {
+    bool was_loaded = false;
+
     float backfire_threshold = 0.0f;
     int backfire_freq = 1;
     int muscle_power_factor = 0;
@@ -130,6 +132,8 @@ struct vpslot_engine {
     int m2c = 100;
     std::vector<std::string> exclusions;
     std::vector<itype_id> fuel_opts;
+
+    void deserialize( const JsonObject &jo );
 };
 
 struct veh_ter_mod {