perf report: Rename 'perf_report' to 'report'

perf report: Rename 'perf_report' to 'report'

Reduce typing, functions use class__method convention, so unlikely to
clash with other libraries.

This actually was discussed in the "Link:" referenced message below.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20131112113427.GA4053@ghostprotocols.net
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>