thunk: introduce split instantiation and payloads

The existing THUNK() instantiation macro has really nice ergonomics
taking the form of the bare function call wrapped by THUNK().

It's desirable however to sometimes enlarge the instance's allocation to
accomodate some user-specified data with an instance-boound life cycle.

There didn't seem to be a clean way to provide that with the existing THUNK()
interface.

This commit introduces a split THUNK() variant for such scenarios:

THUNK_ALLOC(function_name, payload_pointer, payload_size)

and

THUNK_INIT(function_name(thunk_instance, [function_args] ...))

It's expected that these be used in pairs, like so:

	char *buf;
	thunk_t *t = THUNK_ALLOC(foo_func, &buf, BUFSIZ);

	THUNK_INIT(foo_func(t, foo_arg1, foo_arg2));

Note the THUNK_INIT's foo_func call has the thunk_t * passed as the first
parameter.  This isn't part of foo_func's THUNK_DEFINE signature, it's only
part of the initializer for the function, followed by the real parameters
from the THUNK_DEFINE's signature if there are any.

This differs from the simpler THUNK() variant, where you'd do:

	THUNK(foo_func(foo_arg1, foo_arg2));

Also note the THUNK_ALLOC() above has stored a pointer to BUFSIZ bytes
of payload memory @ *buf.  This could then be supplied into foo_func,
which is sometimes convenient:

	THUNK_INIT(foo_func(t, foo_arg1, buf));

The payloads are particularly useful when chaining up multiple thunks, and
there's a need for some shared state across them for inputs/outputs, that
you'd like automatically cleaned up when the instance the payload belongs to
executes and returns freeing itself.

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