py/gc: Add MICROPY_GC_CONSERVATIVE_CLEAR option to always zero memory.

There can be stray pointers in memory blocks that are not properly zero'd
after allocation.  This patch adds a new config option to always zero all
allocated memory (via gc_alloc and gc_realloc) and hence help to eliminate
stray pointers.

See issue #2195.

py/malloc.c

@@ -117,7 +117,10 @@ void *m_malloc0(size_t num_bytes) {
     if (ptr == NULL && num_bytes != 0) {
         return m_malloc_fail(num_bytes);
     }
+    // If this config is set then the GC clears all memory, so we don't need to.
+    #if !MICROPY_GC_CONSERVATIVE_CLEAR
     memset(ptr, 0, num_bytes);
+    #endif
     return ptr;
 }