Non-stream signal reversibility

Currently, it is allowed to specify a reverse signal on the Stream node.
It would be nice to also allow reverse signals on signals that are not streams.

It seems more logical to me that reverse should be a property of a group and union field, as "reverse" is always relative to something else.

Since at the type level, interface modes are (obviously) not exposed, it must be relative to something else _within_ a Logical"Stream"Type, and thus it can only be another group or union field.

If such a group or union element with a reversed field is streamed, the "synthesis" algorithms probably need to be adjusted to create new streams for those specific fields.

Example of a Streamlet Definition File with:
```
Streamlet dolphin {
x : out Stream, b: Bits<1>>>
}
```
Would result in two streams: x.a going out and x.b coming in, both having the same properties of the parent stream.

For unions, if the first field is reversed, e.g.:
```
Streamlet triggerfish {
y : out Stream, b: Bits<1>>>
}
```
Not entirely sure but I think this should result in two streams:

y.(tag & b) going out and may not assume y.a. will send something before sending the tag.
y.a coming in.