FIX tools: Message encoding performance test

Add benchmark for message encoding to understand libtrading performance
better. You can run it with:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 1000000
  encode     1000000 0.264554 µs/message

Signed-off-by: Pekka Enberg <penberg@iki.fi>
lib: Simplify buffer_sum()

Use buffer_sum_range() for buffer_sum() to simplify it.  A nice
side-effect of the simplification is increased encoding speed:

  Before:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 10000000
  encode     10000000 0.214934 µs/message

  After:

  [penberg@localhost libtrading]$ ./tools/fix/fix_perf 10000000
  encode     10000000 0.186174 µs/message

Signed-off-by: Pekka Enberg <penberg@iki.fi>