lib: Simplify buffer_sum()
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 <add4fcd06328a394f0ad91feda7ee057316dc5ed@iki.fi>

Bitcoinsulting/libtrading,svdev/libtrading,divaykin/libtrading,Bitcoinsulting/libtrading,etoestja/libtrading,penberg/libtrading,jvirtanen/libtrading,femtotrader/libtrading,divaykin/libtrading,jvirtanen/libtrading,femtotrader/libtrading,mstanichenko/libtrading,fengzhyuan/libtrading,NunoEdgarGub1/libtrading,etoestja/libtrading,penberg/libtrading,libtrading/libtrading,libtrading/libtrading,svdev/libtrading,fengzhyuan/libtrading,NunoEdgarGub1/libtrading,mstanichenko/libtrading