Build a Bump Arena

Implement a bump allocator over caller-provided memory: ``c typedef struct { unsigned char *mem; size_t cap; size_t used; } arena_t; void arena_init(arena_t *a, unsigned char *buf, size_t cap); void *arena_alloc(arena_t *a, size_t n); void arena_reset(arena_t *a); ` arena_alloc returns NULL when the request does not fit the remaining capacity; arena_reset rewinds used` to 0.