Concat kdf

This is my implementation of Concat KDF. Please ignore the first two commits, they originate from pull request #154 and are better reviewed there.
About testing: I could only find one test vector, in https://tools.ietf.org/html/draft-ietf-jose-json-web-algorithms-37 section C. This test is OK on PLATFORM=vexpress-qemu_virt.