Log Message:
Updated afl to 2.18b.

The lowest currently recommended version is 2.18b.

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Version 2.18b:
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  - Made several performance improvements to has_new_bits() and
    classify_counts(). This should offer a robust performance bump with
    fast targets.

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Version 2.17b:
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  - Killed the error-prone and manual -Z option. On Linux, AFL will now
    automatically bind to the first free core (or complain if there are no
    free cores left).

  - Made some doc updates along these lines.