as_tidygtfs and primary key check

[polettif]

This PR:

• adds a generic as_tidygtfs method and implementations for list, gtfs, dt_gtfs and tidygtfs (as discussed in https://github.com/r-transit/gtfsio/discussions/29)
• implements primary keys for tables and checks wheter they are unique (closes #192)

Other changes:

• removes checkmate from dependencies (was only used in validate_gtfs)
• files added to spec (fare_products, fare_leg_rules, fare_transfer_rules, areas, stop_areas)
• changed link to GTFS reference

[codecov-commenter]

Codecov Report

Merging #198 (d97a64c) into master (11cec60) will decrease coverage by 0.27%. The diff coverage is 99.31%.

:exclamation: Current head d97a64c differs from pull request most recent head 8a6c58e. Consider uploading reports for the commit 8a6c58e to get more accurate results

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@@            Coverage Diff             @@
##           master     #198      +/-   ##
==========================================
- Coverage   99.90%   99.64%   -0.27%     
==========================================
  Files          15       16       +1     
  Lines        1057     1121      +64     
==========================================
+ Hits         1056     1117      +61     
- Misses          1        4       +3     

Impacted Files	Coverage Δ
R/validate_gtfs.R	97.91% <94.11%> (-2.09%)	:arrow_down:
R/tidygtfs.R	96.87% <96.87%> (ø)
R/dates.R	100.00% <100.00%> (ø)
R/io.R	100.00% <100.00%> (ø)
R/spec.R	100.00% <100.00%> (ø)
R/time.R	100.00% <100.00%> (ø)

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