Add sparse array data type.

This commit adds a space and time efficient sparse array data structure.
The structure's raw API is wrapped by inline functions which provide type
safety.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/8197)