stream: STREAM_TYPE_FILE will now read file upfront

No more having to chunk read - if ~stream_init_file~ is used, the
constructor slurps the entire file into the cache.  This pays up front
for a bunch of checks essentially.

~stream_init_pipe~ should be used for chunked reading.

test/test_stream.c

@@ -113,9 +113,8 @@ void stream_test_file(void)
       TEST(err == STREAM_ERR_OK, "Expected initialisating to be okay: %s",
            stream_err_to_cstr(err));
     }
-    TEST(stream_size(&stream) == 0, "Stream doesn't read on init: size = %lu",
-         stream_size(&stream));
     TEST(!stream_eoc(&stream), "Stream should not be at the EoC from init.");
+    stream_stop(&stream);
   }
 
   // try to initialise the stream again but against a nonexistent file - we're