Re-dispatch automatically compiled user natives
Re-dispatch automatically compiled user natives

When no explicit COMPILE was used on a user native, trying to call the
pending native function would trigger a call to COMPILE with default
settings.  Successive calls would work, but that first call didn't
actually invoke the compiled code.

This uses R_REDO_UNCHECKED to achieve that desired first run.
Includes comments to help readers understand what is actually happening.

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