Asset Packs: Optimize icon loader (#164)

* Original pointer can be const

* Back to const icons

* Missed this one

* Simpler string alloc

* Single allocation and header struct for static icons

* Shared allocation and meta struct for animated icons

* Only try to load if dir exists

* Restructure momentum lib

* Use some internal headers

* Swap icons at draw

* Properly init and free, no more original in icon struct

lib/momentum/asset_packs_i.h

@@ -0,0 +1,21 @@
+#include "asset_packs.h"
+
+#include <m-list.h>
+
+typedef struct {
+    const Icon* original;
+    const Icon* replaced;
+} IconSwap;
+
+LIST_DEF(IconSwapList, IconSwap, M_POD_OPLIST)
+#define M_OPL_IconSwapList_t() LIST_OPLIST(IconSwapList)
+
+typedef struct {
+    IconSwapList_t icons;
+    uint8_t* fonts[FontTotalNumber];
+    CanvasFontParameters* font_params[FontTotalNumber];
+} AssetPacks;
+
+extern AssetPacks* asset_packs;
+
+const Icon* asset_packs_swap_icon(const Icon* requested);