ffi: clean up temp allocs on exception.

The problem is that the argument conversions in a FFI call can
bail due to an exception, leaving the argument space partially
initialized, and containing pointers to temporary buffers that
have been allocated with malloc.

In this change, we take care of this problem.

We introduce a new virtual function called release to the FFI
type hieararchy. Most types do not implement this: only types
that have something to free, or aggregates which have to
iterate over element types.

The release virtual relies on pointers being null if nothing
has been allocated. For this reason, we allocate the argument
space and all temporar areas with zero filling.

* ffi.c (zalloca): New macro.
(struct txr_ffi_type): New member function pointer, release.
(ffi_fixed_alloc): Use chk_calloc instead of chk_malloc,
so that any pointers in the area are null.
(ffi_varray_alloc): Use chk_calloc, which simplifies the
code.
(ffi_simple_release, ffi_ptr_in_release, ffi_struct_release):
New static functions.
(ffi_array_release_common, ffi_array_release,
ffi_varray_release): New static functions.
(make_ffi_type_pointer): New argument for specifying release
function.
(make_ffi_type_struct): Initialize release member of type
structure to ffi_struct_release.
(make_ffi_type_array): Initialize release member of type
structure to ffi_array_release.
(ffi_type_compile): Set up release function for varray,
several ptr variants and buf-d.
(ffi_init_types): Set up release function for string types.
(ffi_call_wrap): Internals restructured. Instead of repeatedly
processing type and arg lists, we now store them into arrays
allocated with alloca. We allocate the argument space with
zalloca instead of alloca. Allocating the arguments is
separated from the put conversions into two loops, because
the alloca calls must be outside of the unwind catching
block. This is because the unwind control transfer will roll
back the alloca allocation done since the establishment of
the catch block! A new unwind block now iterates over the
arguments and calls release on each one which has a release
function.

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