Simplify key material exporter backend API

Simplify key material exporter backend API

Just pass pointer and length, instead of a gc and return (possibly)
allocated memory. Saves us some gc instantiations and memcpy()s. Exact
same functionality, 19 lines less code.

(Didn't want to delay the TLS EKM reviews for this, so submitted as a
patch afterwards.)

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20201009144755.39719-1-steffan@karger.me>
URL: https://www.mail-archive.com/search?l=mid&q=20201009144755.39719-1-steffan@karger.me
Signed-off-by: Gert Doering <gert@greenie.muc.de>