ffi: support flexible structures.

This commit adds support for working with foreign structures
which use the C99 "flexible array member", or possibly the
old "C struct hack".  Some improvements are made in the FFI
code the area of incomplete types and bitfields.

* ffi.c (struct txr_ffi_type): New flag members incomplete
and bitfield.
(ffi_varray_dynsize): Check incomplete type flag rather than
for zero size.
(ffi_flex_dynsize, ffi_flex_alloc, ffi_flex_struct_in): New static functions.
(ffi_struct_in): Call ffi_flex_struct_in when doing by-value
semantics on a flexible structure, just before converting last
member. This checks for the length method, calls it, and
adjusts the last member object as documented.
(ffi_struct_get): Likewise.
(bitfield_syntax_p): Static function removed; there is a
flag for this now in the type, so no need to parse its
syntax.
(make_ffi_type_pointer): Test bitfield flag rather than
using bitfield_syntax_p.
(make_ffi_type_struct): Take new argument indicating that the
struct is flexible. Wire in appropriate virtual functions
based on this flag, and set the type's incomplete flag
accordingly. Check bitfield flag rather than using
bitfield_syntax_p for detecting bitfield members.
Calculate the size of flexible struct differently: it is
the offset of the last member.
(make_ffi_type_union): Test bitfield flag rather than
using bitfield_syntax_p.
(ffi_struct_compile): Renamed to ffi_membs_compile.
(ffi_membs_compile): New pointer parameter for returning an
indication whether the member list specifies a flexible
struct. Test the incomplete flag of a member type rather than
size being zero.
(ffi_type_compile): Follow rename of ffi_struct_compile to
ffi_membs_compile. Pass down the flag obtained from
ffi_membs_compile into make_ffi_type_struct so that a flexible
struct type is created when necessary.  Disallow arrays of
bitfields.  Set bitfield flag on bitfield types.
(ffi_init_types): Set incomplete flag on void type.
(ffi_make_call_desc): Test for incomplete and bitfield types
using new flags.
(ffi_typedef, ffi_size, ffi_alignof): Test for bitfield type
using new bitfield flag.
(carray_elem_check): New static function.
(carray_buf, carray_pun, carray_unum, carray_num): Use new
carray_elem_check to check for bad array element type. This
uses the incomplete flag rather than checking for zero size,
and also disallows bitfields.

* lib.h (length_s): Declared.

* txr.1: Flexible structs documented in new section. Also
section on incomplete types. Description of sizeof operator
gives more detaild about incomplete types including flexible
structs.

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