Add header union information for extract operator

[VolodymyrPeschanenkoIntel]

This small changes add information for extract operator then current header is a part of header union stack like in example from this PR.
For example:

header O1 {
    bit<8> data;
}

header O2 {
    bit<16> data;
}

header_union U {
    O1 byte;
    O2 short;
}

struct headers {
    S    base;
    U[2] u;
}
....
parser ParserImpl(packet_in packet, out headers hdr, inout metadata meta, inout standard_metadata_t standard_metadata) {
    state start {
        packet.extract(hdr.u.next.byte);

[mihaibudiu]

I think this will need more changes, in particular modifying the pseudocode in the green blocks.

[mihaibudiu]

But this is a good start to have the design committee discuss this problem.

[jafingerhut]

LGTM