Introduce bk_err and bk_bool rather than using int for both of these.
This means that the function definition now specifies whether 0 means
no error, or whether it means false.
Valgrind previously did not fail the test suite when there were memory
leaks or invalid accesses. Due to this, a few bugs slipped in. These bugs
have been fixed, and now every container has more test cases.
Furthermore, the test suite has been updated to fail if Valgrind reports
any errors.
