c-ext: change casts between Py_ssize_t and size_t

In all these cases, we are comparing a Py_ssize_t (from Python
obviously) with a size_t in zstd. The Py_ssize_t type is signed.
But in these cases, the value refers to the length of a Py_buffer.
The length of a Py_buffer can never be negative. And the size
of Py_ssize_t must be the same as size_t. So it follows that casting
a Py_buffer to size_t should be safe.

This avoids the use of ssize_t in our code. And this should hopefully
unblock building the extension with PyPy on Windows.

Fixes #61.

NEWS.rst

@@ -59,6 +59,11 @@ Bug Fixes
 
 * ``zstd_cffi.py`` added to ``setup.py`` (#60).
 
+Changes
+-------
+
+* Change some integer casts to avoid ``ssize_t`` (#61).
+
 0.10.1 (released 2018-10-08)
 ============================
 

c-ext/compressionwriter.c

@@ -153,7 +153,7 @@ static PyObject* ZstdCompressionWriter_write(ZstdCompressionWriter* self, PyObje
 	input.size = source.len;
 	input.pos = 0;
 
-	while ((ssize_t)input.pos < source.len) {
+	while (input.pos < (size_t)source.len) {
 		Py_BEGIN_ALLOW_THREADS
 		zresult = ZSTD_compress_generic(self->compressor->cctx, &output, &input, ZSTD_e_continue);
 		Py_END_ALLOW_THREADS

c-ext/compressobj.c

@@ -59,7 +59,7 @@ static PyObject* ZstdCompressionObj_compress(ZstdCompressionObj* self, PyObject*
 	input.size = source.len;
 	input.pos = 0;
 
-	while ((ssize_t)input.pos < source.len) {
+	while (input.pos < (size_t)source.len) {
 		Py_BEGIN_ALLOW_THREADS
 			zresult = ZSTD_compress_generic(self->compressor->cctx, &self->output,
 				&input, ZSTD_e_continue);

c-ext/decompressionwriter.c

@@ -93,7 +93,7 @@ static PyObject* ZstdDecompressionWriter_write(ZstdDecompressionWriter* self, Py
 	input.size = source.len;
 	input.pos = 0;
 
-	while ((ssize_t)input.pos < source.len) {
+	while (input.pos < (size_t)source.len) {
 		Py_BEGIN_ALLOW_THREADS
 		zresult = ZSTD_decompress_generic(self->decompressor->dctx, &output, &input);
 		Py_END_ALLOW_THREADS