py/gc: Make GC block size be configurable.

py/gc.c

@@ -46,8 +46,8 @@
 // make this 1 to dump the heap each time it changes
 #define EXTENSIVE_HEAP_PROFILING (0)
 
-#define WORDS_PER_BLOCK (4)
-#define BYTES_PER_BLOCK (WORDS_PER_BLOCK * BYTES_PER_WORD)
+#define WORDS_PER_BLOCK ((MICROPY_BYTES_PER_GC_BLOCK) / BYTES_PER_WORD)
+#define BYTES_PER_BLOCK (MICROPY_BYTES_PER_GC_BLOCK)
 
 // ATB = allocation table byte
 // 0b00 = FREE -- free block

py/mpconfig.h

@@ -96,6 +96,10 @@
 /*****************************************************************************/
 /* Memory allocation policy                                                  */
 
+// Number of bytes in memory allocation/GC block. Any size allocated will be
+// rounded up to be multiples of this.
+#define MICROPY_BYTES_PER_GC_BLOCK (4 * BYTES_PER_WORD)
+
 // Number of words allocated (in BSS) to the GC stack (minimum is 1)
 #ifndef MICROPY_ALLOC_GC_STACK_SIZE
 #define MICROPY_ALLOC_GC_STACK_SIZE (64)