Keep original readline buffer sizes

When we create a Readline "context" object we heap allocate buffers that
libreadline requires. So far we have specified our own buffer sizes,
independent of libreadline's defaults. But there really is no good
reason not to go with whatever they were using from the get-go.
Thus, with this change we just allocate as many bytes as libreadline
would by default.

src/lib.rs

@@ -250,10 +250,8 @@ impl Readline {
         // allocated by libreadline itself, as part of its
         // initialization. Because we create a new context we need to
         // reinitialize this data.
-        rl_line_buffer = calloc(1, 64).cast();
-        rl_line_buffer_len = 64;
-        rl_executing_keyseq = calloc(1, 16).cast();
-        rl_key_sequence_length = 16;
+        rl_line_buffer = calloc(1, rl_line_buffer_len as _).cast();
+        rl_executing_keyseq = calloc(1, rl_key_sequence_length as _).cast();
 
         // We use similar behavior to default Rust and panic on
         // allocation failure.