Sequence: use field reference instead of type specifier

Sequence can be made more flexible by specifying its size using a
reference to an unsigned integer field (declared prior to the sequence)
rather than specifying the type. Many sequences can now share the same
length field, and the length field does not have to be physically
contiguous with the sequence.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>