Make a faster List.pick(*,:eager) version
Make a faster List.pick(*,:eager) version

Well, that was the idea.  Turns out the eager version is about 20x *slower*.
Added here for further research.

What has been ruled out:
- anything inside the loop
  That part of the code is mostly the same as the non-eager version.  If
  the code inside the while loop is reduced to just the $elems = $elems - 1,
  even then the eager variant is about 15x slower.
- something in the MMD dispatch
  If the code inside the "eager" variant is the same as the non-eager variant,
  there is no difference in execution noticeable either.
- not SPESH related
  With MVM_SPESH_DISABLE=1 the difference in execution is even larger: the
  optimizer did its best!

What *is* different?
- the number of frames in a profile
  example code: my @a = ^100; for ^500 { @a.pick(*) } # or *, :eager
  non-eager / eager: 10618 / 176614 frames
  So the difference in number of frames is in same order of magnitude as the
  difference in CPU.

So the question is really: why does the eager version create so many frames?

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