implied yield with comma-separated expressions

[jameskerr]

It would be nice if implied yield worked for comma-separated expressions a la

... | {a:this},{b:this} | ...

Moved from: https://github.com/brimdata/brim/issues/2161

[philrz]

Still with us as of Zed commit 9c0f097.

$ zq -version
Version: v1.2.0-49-g9c0f0973

$ seq 1 | zq -z 'yield {a:this},{b:this}' -
{a:1}
{b:1}

$ seq 1 | zq -z '{a:this},{b:this}' -
zq: error parsing Zed at column 9:
{a:this},{b:this}
    === ^ ===