Add a basic specification for the olm protocol and format.

Replace hard coded references to the 32-byte key length with a constant, add utilities for copying data to and from fixed sized arrays

Initialise the length fields of the reader struct in decode_message, even if the message is invalid, fixes a crash where the message was too short

Add versions of olm_session_create_inbound and olm_session_matches_inbound which take the curve25519 identity key of the remote device we think the message is from as an additional argument

Add a test of olm which uses malloc to create it's buffers so that it is possible to check of out of bounds reads and writes using valgrind

Fix bug in list where the wrong value was copied if an item was inserted at the beinging of the list

Call the right c function from Session.matches_inbound. It was calling create_inbound_session rather than matches_inbound_session