This adds the ability to consume perf events in libbpf/python using the
ring buffer. For now, this is the only way to get access to the function
call graph. Only kernel functions are supported.

It does this by introducing a new set of libbpf helper functions that
can open the perf fd, mmap it, and poll over the events as they are
submitted by the kernel. This allow for faster event processing than
trace_printks, but has not been tested.

The functionality is disabled by default, the user can enable it by
passing a non-empty cb parameter into the BPF constructor. That cb
function will be invoked for each event that is read from the buffer.
Buffers are per-fd, so each event is distinct as well as separated from
other processes that may be running simultaneously.

The initial test case uses this functionality to build a histogram of
events keyed by the callchain.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>