Fix a fairly serious bug whereby Vec's could incorrectly compare as equal (#204)

* chiselTests: include an example of two empty Vectors killing FIRRTL

* Aggregate: fix a bug whereby Vec[T] was using equals/hashCode of Seq

In Chisel, two vectors are NOT equal just if their contents are equal.
For example, two empty vectors should not be considered equal. This
patch makes Vec use the HasId._id for equality like other Chisel types.

Without this fix, Bundle.namedElts.seen: HashSet[Data]() will eliminate
one of the named vectors and emit bad IR.