zq update through "allow typedefs to be redefined" by mccanne

[github-actions]

This is an auto-generated PR with a zq dependency update needing manual attention. brimdata/super#2386, authored by @mccanne, has been merged, however zq could not be updated automatically. Please see https:/brimsec/brim/actions/runs/677403237 for the original failing run. If a Brim update is needed, you may use the branch on this PR to do so.

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allow typedefs to be redefined

This commit relaxes the constraint that type names cannot be redefined
to a different type. This follows the Z design principle of dealing
with whatever the user or client throws at the system as best we can.

The problem of schema versioning is outside the scope of the ZSON
format and can be solved at a higher layer with the appropriate Z
building blocks.

We also removed the end-of-sequence concept from the ZSON spec as
it is not relevant here as the definitions of type contexts and
redefinable types allows an implementation to create a seekable output.

Closes brimdata/super#1568

[philrz]

The CI passed on a subsequent zq commit which advanced the pointer, so this run may have failed due to an intermittent bug. #1529 has been opened to track that.