vm: add alignment parameter to cache allocator

caches with object size N, where N is a power of two, will naturally
align their objects on N-byte boundaries.

all other caches will align to an 8-byte boundary by default.

include/socks/util.h

@@ -2,7 +2,9 @@
 #define SOCKS_UTIL_H_
 
 #include <stddef.h>
+#include <stdbool.h>
 
 extern void data_size_to_string(size_t value, char *out, size_t outsz);
+static inline bool power_of_2(size_t x) { return (x > 0 && (x & (x - 1)) == 0); }
 
 #endif