Add "peek" feature, similar to that of Java Streams API

[kchro3]

Here is a proposal for an idiomatic way to do a "peek" operation (https://www.baeldung.com/java-streams-peek-api).

TL;DR: In Java, you can call .peek( someFn ) as an intermediate step in the pipeline, but it wouldn't modify the value like .map would.

# for example
(
  seq(1, 2, 3)
  .map(lambda x: x ** 2)
  .peek(print)
  .to_set()
)

I think it could be helpful for logging / maintaining stats / debugging purposes.

[codecov[bot]]

Codecov Report

Merging #166 (13df3cb) into master (d757a90) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   97.98%   98.00%   +0.01%     
==========================================
  Files          12       12              
  Lines        2237     2253      +16     
==========================================
+ Hits         2192     2208      +16     
  Misses         45       45              

Impacted Files	Coverage Δ
functional/pipeline.py	98.09% <100.00%> (+<0.01%)	:arrow_up:
functional/test/test_functional.py	98.94% <100.00%> (+<0.01%)	:arrow_up:
functional/transformations.py	100.00% <100.00%> (ø)

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[EntilZha]

Thanks for the PR!