Minor pseudovector allocation cleanups

Minor pseudovector allocation cleanups

* src/alloc.c (VECTOR_BLOCK_SIZE, VECTOR_BLOCK_BYTES)
(VBLOCK_BYTES_MIN, VBLOCK_BYTES_MAX, VECTOR_MAX_FREE_LIST_INDEX):
Prefer enums to macros where either will do.
(allocate_vector_from_block): Arg is ptrdiff_t, not size_t.
Use eassume instead of eassert.
(PSEUDOVEC_STRUCT): New macro, which verifies the already-existing
assumption that the vector-like objects are small.
(cleanup_vector): Use it.  Use if-then-else systematically;
this lets GCC do a bit better job.

2018-08-08  Paul Eggert  <eggert@cs.ucla.edu>

* src/alloc.c (VBLOCK_BYTES_MAX): Use vroundup_ct, not
vroundup, so that can be used in static assertions.