erasure-code: move jerasure tests to a subdirectory

Along with the associated Makefile to make room in the test/osd
directory and prepare for other plugins.

Signed-off-by: Loic Dachary <loic@dachary.org>
decode/encode simplified interface

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

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