ffi: bitfield tests and fixes.

The bitfield allocation rules are wrong. Some of it is due
to the recent changes which are based on incorrect analysis,
but reverting things doesn't fix it.

The idea that we compare the current member's alignment
with the previous is wrong; it is not borne out by empirical
tests with gcc. So we do a straight revert of that.

In GNU C, an __attribute__((aligned (N))) attribute applied
to a bitfield member will perform the requested alignment if,
evidently, the bit field is already being placed into a new
byte. (If the bit field is about to be packed into an existing
byte, then there is a warning about the align attribute being
ignored). Because we don't have alignment as a member attribute,
but only as a type attribute, we must implement a flag which
indicates that a type has had align applied to it (even if
the alignment didn't change) so we can then honor this in the
right place in the bitfield allocation code.

* ffi.c (struct txr_ffi_type): New attribute flag, aligned.
(make_ffi_type_struct): Remove the prev_align variable and
all related logic.  Consolidate all alignment into one place,
which is done before we allocate the bitfield or regular member.
We align if the new member isn't a bitfield, or even if it is
a bitfield if it has the aligned attribute, or if the bitfield
is changing endian compared to the previous member (our local
rule, not from GNU C).
(ffi_type_compile): The align and pack operators now set the
aligned attribute, except in the (pack 1 ...) case which
semantically denotes lack of alignment.

* tests/017/bitfields.tl: New file.

* txr.1: Documented.

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