v2.1.1

This release fixes a bug where the empty ‘h’ was permitted as the first
operation in a pattern:

    h//  # No previous regex to snag!

Along with making no sense, this also resulted in an out-of-bounds index
bug.  Attempting to perform this now results in a diagnostic message:

    ⇒ h//
    grab: Empty ‘h’ is not allowed as the first operator
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