Remove Applicative#pureEval

[ceedubs]

This probably resolves #1150, though it doesn't provide an alternative
type class as was brought up there.

@adelbertc has gotten tripped up by pureEval defaulting to a call to pure with .value called on the Eval.

@johnynek has also questioned pureEval being directly on Applicative.

@tpolecat has raised concerns that pureEval looks like it might exist as a way to inject
side effects into a type, but that it doesn't actually make any claims
about execution semantics or value retention, so it can't be relied upon
to do this.

I think that there could be an argument for having an instantiation method for
a side-effect-y type (like a Task) that takes an Eval[A] and produces an instance
that will repeat the computation given Always, memoize the value given
Later, etc. However, I don't think that @tpolecat necessarily agrees
with me, and I think that even if this is a reasonable thing to do,
Applicative probably isn't the place for it.

Since several people have questioned pureEval, it was entirely
untested, and I haven't heard any one argue for it staying, I'm inclined
to remove it. If someone has a compelling example of when a non-strict
argument can be useful for pure, then I may be convinced. But even
then we should clearly document what behavior can be expected of
pureEval.

@non I believe that you originally added pureEval as part of your StreamingT work. Do you have any thoughts on this?

[codecov-io]

Current coverage is 89.77% (diff: 100%)

Merging #1234 into master will increase coverage by 0.14%

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+ Misses          326        321     -5   
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[johnynek]

👍

[tpolecat]

👍

[adelbertc]

👍

[kailuowang]

👍

[kailuowang]

I apologize @ceedubs and @non, I forgot that @non is mentioned here as well, my bad. I will do the fix if needed.