Rework user ops

[mattnibs]

Rework user ops in the following ways:

• User ops take either a const or path as an argument.
• The this value of the parent is carried through when a user op is invoked.

Fixes #4650
Fixes #4658
Fixes #4653

[philrz]

FWIW, I tested out this branch at commit 6fb851c and confirmed it seems to address what's described in #4650. That is, since we're no longer using spread to pass this into the user op, now primitive values make it through ok.

$ zq -version
Version: v1.8.1-32-g6fb851c9

$ cat kind-in-op-4663.zed
op CallMe(): (
  yield {kind: kind(this),ThisWas: this}
)
CallMe()

$ echo '{foo: "bar"} "hello"' | zq -z -I kind-in-op-4663.zed -
{kind:"record",ThisWas:{foo:"bar"}}
{kind:"primitive",ThisWas:"hello"}

[philrz]

It looks like #4658 is addressed as well. I can now pass in a dotted reference to a nested field or use field referencing with indexing and then reference the field ok from inside the user op.

$ zq -version
Version: v1.8.1-32-g6fb851c9

$ cat yield-in-op-4663.zed
op CallMe(f): (
  yield f
)

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-4663.zed 'CallMe(foo.bar)' -
"baz"

$ echo '{foo: {bar: "baz"}}' | zq -Z -I yield-in-op-4663.zed 'CallMe(this["foo"]["bar"])' -
"baz"

[nwt]

The stuff in runtime/op/scope.go is now unused, so you can remove that.