Don't buffer in IoWrite(), indicate that fwrite() returns size_t, not ssize_t.

TODO.txt

@@ -14,8 +14,8 @@ Milestone: v0.3.0
 [~] Stream API
     [~] Implementation
     [ ] Convert all code that deals with I/O
-    [ ] Multi-output (proof of concept)
-    [ ] Memory streams (proof of concept)
+    [!] Multi-output (proof of concept)
+    [!] Memory streams (proof of concept)
     [ ] TLS
     [ ] SOCKS
     [x] Move/convert UtilStreamCopy()

src/Io/IoFile.c

@@ -37,8 +37,15 @@ static ssize_t
 IoWriteFile(void *cookie, void *buf, size_t nBytes)
 {
     FILE *fp = cookie;
+    size_t res = fwrite(buf, 1, nBytes, fp);
 
-    return fwrite(buf, 1, nBytes, fp);
+    /*
+     * fwrite() may be buffered on some platforms, but at this low level,
+     * it should not be; buffering happens in Stream, not Io.
+     */
+    fflush(fp); 
+
+    return res;
 }
 
 static off_t