feat: Implement pivot_*() functions

[nathaneastwood]

Closes #98.

@etiennebacher I made some modifications to the original PR:

1. I squashed the commits into one.
2. I fixed some syntax issues.
3. I removed the tidyr references. I understand the desire to include them and it kind of makes sense, however there would be a couple of issues doing so:
   a. Namespace clashes - as you pointed out.
   b. I would need to keep up with any changes in the tidyr package which, whilst I aspire to do so, I do not have the time for.
4. Removed any tibble references. poorman isn't designed to work with tibbles.

Re your comment about NSE. Generally I use poorman:::eval_select_pos(). I will look to update this at some point to use it.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@af2940e). Click here to learn what that means.
The diff coverage is 82.64%.

@@            Coverage Diff            @@
##             master     #101   +/-   ##
=========================================
  Coverage          ?   93.23%           
=========================================
  Files             ?       56           
  Lines             ?     1374           
  Branches          ?        0           
=========================================
  Hits              ?     1281           
  Misses            ?       93           
  Partials          ?        0           

Impacted Files	Coverage Δ
R/pivot_wider.R	77.77% <77.77%> (ø)
R/pivot_longer.R	89.79% <89.79%> (ø)

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