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

tests/.gitignore

@@ -6,6 +6,7 @@ ostree-http-server
 run-apache
 tmpdir-lifecycle
 test-rollsum
+test-bloom
 test-bsdiff
 test-checksum
 test-gpg-verify-result