Support dynamic paths for put and cut op

[mattnibs]

Fixes #4555

[philrz]

As a sign that we're on the right track, here's verification that this branch at commit 271bb18 seems to be addressing the use case laid out in #4555. As shown there, creating a new record field with a name derived from a string value required me to use a record literal + unflatten() idiom, which I've never found super intuitive.

$ zq -Z 'over this | ports:=collect(ports[0]) by ip | unflatten([{key:ip,value:ports}])' scan.json | tee 1
{
    "192.168.5.53": [
        {
            port: 515,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        },
        {
            port: 21,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        }
    ]
}
{
    "192.168.5.49": [
        {
            port: 49152,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        },
        {
            port: 3401,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        }
    ]
}

Running this branch at commit 271bb18, I can now obtain the same output using the syntax below, which previously triggered the illegal left-hand side of assignment error.

$ zq -version
Version: v1.10.0-5-g271bb189

$ zq -Z 'over this | ports:=collect(ports[0]) by ip | put this[ip]:=ports | drop ip,ports' scan.json | tee 2
{
    "192.168.5.53": [
        {
            port: 515,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        },
        {
            port: 21,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        }
    ]
}
{
    "192.168.5.49": [
        {
            port: 49152,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        },
        {
            port: 3401,
            proto: "tcp",
            status: "open",
            reason: "syn-ack",
            ttl: 64
        }
    ]
}

$ diff 1 2
[no output]

[mattnibs]

Thanks @philrz.

Btw, I didn't realize this until working on the pr but

over this | ports:=collect(ports[0]) by ip | put this[ip]:=ports | drop ip,ports

can be shortened to

over this | ports:=collect(ports[0]) by ip | cut this[ip]:=ports

[nwt]

!!!