erasure-code: move to ErasureCode::{encode,decode}{,chunk}

erasure-code: move to ErasureCode::{encode,decode}{,chunk}

The bulk of ErasureCode{Jerasure,Isa}::encode and
ErasureCode{Jerasure,Isa}::decode is dealing with alignment and buffer
allocation. This is moved to ErasureCode::encode and ErasureCode::decode
respectively.

The jerasure/isa specific code is isolated in the new
ErasureCode{Jerasure,Isa}::encode_chunks and
ErasureCode{Jerasure,Isa}::decode_chunks virtual functions.

http://tracker.ceph.com/issues/8496 Refs: #8496

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