implement functions in Releases endpoint

[sboysel]

Addresses #23

• The structure of fredr_release_tables()is non-tabular so I opted to return a list instead of a tibble with nested objects. Might not be perfectly "tidy".
• Functions are in separate files and have distinct doc pages. I chose this since some of the parameters change across functions.

[codecov-io]

Codecov Report

Merging #33 into master will increase coverage by 0.57%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
+ Coverage   99.23%   99.81%   +0.57%     
==========================================
  Files          12       21       +9     
  Lines         393      538     +145     
==========================================
+ Hits          390      537     +147     
+ Misses          3        1       -2

Impacted Files	Coverage Δ
R/validation.R	99.21% <100%> (+0.13%)	⬆️
R/fredr_release_dates.R	100% <100%> (ø)
R/fredr.R	100% <100%> (+3.33%)	⬆️
R/fredr_release_sources.R	100% <100%> (ø)
R/fredr_release_related_tags.R	100% <100%> (ø)
R/fredr_releases.R	100% <100%> (ø)
R/fredr_release_tags.R	100% <100%> (ø)
R/fredr_release.R	100% <100%> (ø)
R/fredr_release_tables.R	100% <100%> (ø)
R/fredr_release_series.R	100% <100%> (ø)
... and 11 more

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