refactor: change to lambda functions in purrr, replace vapply w/ purr…

[m-muecke]

Closes https://github.com/tidyfun/tf/issues/48

• Replace formula syntax w/ lambda functions in purrr calls
• Make use of lengths function and remove vapply(x, length, numeric(1) pattern
• Apply styler formatting to some files
• Replace vapply with purrr functions
• Make use of setNames when exiting a function or in a pipe
• Replace unlist map pattern for map_*

[codecov-commenter]

Codecov Report

Attention: 64 lines in your changes are missing coverage. Please review.

Comparison is base (d4e5d65) 69.46% compared to head (9072c80) 69.00%.

:exclamation: Current head 9072c80 differs from pull request most recent head 701f16c. Consider uploading reports for the commit 701f16c to get more accurate results

Files	Patch %	Lines
R/print-format.R	0.00%	16 Missing :warning:
R/smooth.R	0.00%	12 Missing :warning:
R/methods.R	35.29%	11 Missing :warning:
R/tfd-class.R	74.35%	10 Missing :warning:
R/tfb-spline-utils.R	93.54%	4 Missing :warning:
R/calculus.R	66.66%	3 Missing :warning:
R/utils.R	77.77%	2 Missing :warning:
R/vctrs.R	85.71%	2 Missing :warning:
R/brackets.R	80.00%	1 Missing :warning:
R/math.R	50.00%	1 Missing :warning:
... and 2 more

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #61 +/- ## ========================================== - Coverage 69.46% 69.00% -0.46% ========================================== Files 26 26 Lines 1634 1623 -11 ========================================== - Hits 1135 1120 -15 - Misses 499 503 +4 ```

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[fabian-s]

Fantastic, thank you!