Add guard to Alternative

[SystemFw]

This PR adds guard, a small function on Alternative which I personally find pretty useful.

guard: Boolean => F[Unit]

Which returns Alternative[F].empty if the the boolean is false, and ().pure[F] if true

[LukaJCB]

LGTM, thanks! :)

[codecov-io]

Codecov Report

Merging #1939 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1939      +/-   ##
==========================================
+ Coverage   95.55%   95.55%   +<.01%     
==========================================
  Files         248      248              
  Lines        4451     4454       +3     
  Branches      118      121       +3     
==========================================
+ Hits         4253     4256       +3     
  Misses        198      198

Impacted Files	Coverage Δ
core/src/main/scala/cats/Alternative.scala	88.88% <100%> (+1.38%)	⬆️
core/src/main/scala/cats/syntax/alternative.scala	100% <100%> (ø)	⬆️

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