lib/bloom: Add an internal bloom filter implementation

lib/bloom: Add an internal bloom filter implementation

This will be used in an upcoming commit. It adds a basic bloom filter
implementation, using the SipHash family of hash functions.

The implementation (including its parameter choices and hash functions)
will become a protocol detail in future, so must not be changed so that
its output is bitwise incompatible between OSTree versions.

Unit tests are included.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #924
Approved by: cgwalters