Make Typeable macros not dependent on default Predef presence

[MateuszKubuszok]

Typeable macros used classOf directly - it breaks if one uses settings like -Yno-predef or similar (#860). This PR should fix it.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@75454d9). Click here to learn what that means.
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@@           Coverage Diff            @@
##             master    #861   +/-   ##
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  Coverage          ?   87.9%           
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  Files             ?      65           
  Lines             ?    1497           
  Branches          ?       5           
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  Hits              ?    1316           
  Misses            ?     181           
  Partials          ?       0

Impacted Files	Coverage Δ
core/src/main/scala/shapeless/typeable.scala	0% <ø> (ø)

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[joroKr21]

LGTM but needs rebase 👍

More hygiene is always better.

[joroKr21]

@MateuszKubuszok hope you don't mind I rebased the PR myself.

Thank you for the fix! 👍

[MateuszKubuszok]

No problem, thanks! I'm on vacation so I don't look at github very often.