bind: use golang.org/x/mobile/bind/seq for cross-language call

[sbinet]

gopy currently (mostly) sends pointers to go values back and forth between cgo and go.
on the cpython side, such types are (usually) defined like so:

typedef void *cgo_type_pkg_MyType;

struct cpy_type_pkg_MyType {
  PyObject_HEAD
  cgo_type_pkg_MyType cgopy; /* pointer to pkg.MyType */
};

under the rules currently defined at golang/go#12416, this will no longer work as we store a pointer to go memory, possibly containing itself pointers to go memory.

so we'll probably have to bite the bullet and move to the (de)serialization protocol of golang.org/x/mobile/bind/seq.

[rcoreilly]

for old version, closing