Shuffle around wall IDs, change wall check

Previously, walls were all numbered at 21000+. This is bad because
our blockmap is an array, so this caused our blockmap to take up
80 MiB of RAM in each worker process.

This commit changes the wall numbering, and exploits some bitmasking
to have the wall check run in constant time no matter how many walls
there are. This is done with a simple mask and xor to check the prefix.

RAM usage for the blockmap thus drops to like 44 MiB.

This is in preparation for adding more walls for 1.16.

overviewer_core/src/block_class.c

@@ -58,6 +58,13 @@ bool block_class_is_subset(
     return false;
 }
 
+bool block_class_is_wall(mc_block_t block) {
+    mc_block_t mask = 0b11111111;
+    mc_block_t prefix = 0b111 << 8;     // 1792 is the starting offset
+    // if the xor zeroes all bits, the prefix must've matched.
+    return (block & ~mask) ^ prefix == 0;
+}
+
 const mc_block_t block_class_stair[] = {
     block_oak_stairs,
     block_brick_stairs,
@@ -252,20 +259,3 @@ const mc_block_t block_class_alt_height[] = {
     block_purpur_slab,
     block_wooden_slab};
 const size_t block_class_alt_height_len = COUNT_OF(block_class_alt_height);
-
-const mc_block_t block_class_wall[] = {
-    block_andesite_wall,
-    block_brick_wall,
-    block_cobblestone_wall,
-    block_diorite_wall,
-    block_end_stone_brick_wall,
-    block_granite_wall,
-    block_mossy_cobblestone_wall,
-    block_mossy_stone_brick_wall,
-    block_nether_brick_wall,
-    block_prismarine_wall,
-    block_red_nether_brick_wall,
-    block_red_sandstone_wall,
-    block_sandstone_wall,
-    block_stone_brick_wall};
-const size_t block_class_wall_len = COUNT_OF(block_class_wall);