Improve duplicated primary key check

[polettif]

As it turns out, the implemented check for duplicated primary keys is very slow on larger feeds. This PR improves the runtime by about 10x (!) by using data.table::anyDuplicated. Example benchmarks:

With the NYC dataset

nyc = gtfsio::import_gtfs(system.file("extdata", "google_transit_nyc_subway.zip", package = "tidytransit"))
duplicated_primary_keys(nyc)

Unit: milliseconds
 expr       min       lq      mean    median        uq      max neval
  old 531.28157 624.1847 673.41524 696.80770 714.99092 763.3479    20
  new  26.70495  30.0662  43.68424  33.98627  46.30624 106.0321    20

With the Swiss dataset:

Unit: seconds
 expr       min        lq      mean    median        uq       max neval
  old 16.113029 17.393730 18.626484 17.964645 20.772904 20.888113     5
  new  1.063265  1.117334  1.569431  1.244815  1.675949  2.745791     5

Maybe there's faster ways to check for duplicated keys but I can't think of a better way currently. Checking for unique keys in large datasets is bound to come with some cost.

[codecov-commenter]

Codecov Report

Merging #203 (d6924d2) into master (18cf9b4) will increase coverage by 0.00%. The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #203   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files          16       16           
  Lines        1119     1120    +1     
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+ Hits         1118     1119    +1     
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Impacted Files	Coverage Δ
R/validate_gtfs.R	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes