remove `var` for ref parameters in `std/tables`

[ringabout]

ref https://github.com/nim-lang/Nim/pull/2139

It seems that it was probably caused by copy pasting.

[ringabout]

The PR makes hasKeyOrPut accept immutable refs like mgetOrPut does. It should does less harm in breaking changes' regard. Though it can probably break something if people pass it to function parameters.

No other procs in tables disallow immutable refs.

[github-actions[bot]]

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