Add support for variable-length BIGINT

This commit introduces a variable-length integer that offers space
savings when used with columns that usually contain smaller values but
must support larger ones.  It implements this similar to VARBINARY,
i.e., a length prefix byte followed by 0-8 payload bytes.

TODO: factor out and unit test VARINT parsing and packing
TODO: store small values in length byte?
TODO: how does this interact with legacy BIGINT?
TODO: large scale testing
TODO: unsigned integers
TODO: remove debug logs
TODO: announce on InnoDB mailing list