Replace Malloc With Pools

This commit introduces an object pool and replaces instances of malloc with pools where sensible.

source/pool.c

@@ -0,0 +1,30 @@
+#include "pool.h"
+
+void pool_clear_idx(PoolBitmap *bm, int idx) {
+    u32 i = idx / POOL_BITS_PER_WORD; // word offset
+    u32 b = idx % POOL_BITS_PER_WORD; // bit offset
+    bm->w[i] &= ~((POOL_WORD_T)1 << b);
+}
+
+int pool_get_free_idx(PoolBitmap *bm) {
+    for (u32 i = 0; i < bm->nwords; i++) {
+        POOL_WORD_T inv = ~bm->w[i];
+
+        // guard so we don't cal `ctz` with 0, since __builtin_ctz(0) is undefined
+        // https://gcc.gnu.org/onlinedocs/gcc/Bit-Operation-Builtins.html#index-_005f_005fbuiltin_005fctz
+        //
+        // By using the bitwise inverse of the word, you can skip words that are full
+        // quickly (where the value is 0 or 'false' since all bits are '1', or 'in use').  Any value greater
+        // than 0 indicates there is a free slot. Then, when counting the trailing 0's, you can test very quickly
+        // where the first free slot is. This operation prevents looping through every bit of filled flags, and
+        // will instead operate only on the first word with free slots.
+        if (inv) {
+            int bit = __builtin_ctz(inv);
+            bm->w[i] |= ((POOL_WORD_T)1 << bit);
+            int idx = i * POOL_BITS_PER_WORD + bit;
+            return idx;
+        }
+    }
+
+    return -1;
+}