5f115023

By: Tom Sydney Kerckhove <syd@cs-syd.eu>

Fix exponential-time blowup in many/some parsing

A parser shaped like

    choice [some argument, many --filter] <*> many --opt1 <*> many --opt2

scales exponentially in the number of @--opt2@ values when there are
also many @--opt1@ values: every added @--opt2@ multiplies total parse
time by ~3x. Originally hit by a sydtest mutation suite invocation with
6 @--mutation@ + 6 @--mutation-suite-exe@ flags taking >30 seconds of
CPU just to parse settings.

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