vec: vec_try_append

Essentially a method to attempt to append data but without doing any
reallocation - stay within the bounds of the capacity.

include/alisp/vec.h

@@ -36,7 +36,10 @@ void vec_init(vec_t *, u64);
 void vec_free(vec_t *);
 u8 *vec_data(vec_t *);
 
+// Append, possibly reallocating memory
 void vec_append(vec_t *, const void *const, u64);
+// Try to append without allocating memory
+bool vec_try_append(vec_t *, const void *const, u64);
 void vec_ensure_free(vec_t *, u64);
 void vec_clone(vec_t *, vec_t *);