erasure-code: add chunk size calculation helpers

With the current erasure code interface, figuring out the size of a
chunk and the number of chunks for a given object can only be achieved
by calling the encode() method on an actual buffer.

The get_chunk_count and get_chunk_size helpers are added and documented.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: refactor the example to use chunk size helpers

The get_chunk_size and get_chunk_count methods are implemented using the
code found in the encode() method. The encode method is modified to use
the chunk size helpers.

The unit tests are modified to use the helpers.

Signed-off-by: Loic Dachary <loic@dachary.org>
erasure-code: jerasure stub for chunk size helpers

The implementation is delayed to avoid conflicts with the basic
pyramidal code implementation.

Signed-off-by: Loic Dachary <loic@dachary.org>