Add wk_crs_proj_definition generic()

[paleolimbot]

Fixes #110. Also uses the PROJ definition when printing!

[codecov-commenter]

Codecov Report

Merging #112 (7cc0523) into master (7bd345a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage   98.50%   98.52%   +0.01%     
==========================================
  Files          71       71              
  Lines        4935     5000      +65     
==========================================
+ Hits         4861     4926      +65     
  Misses         74       74              

Impacted Files	Coverage Δ
R/class-sf.R	99.13% <100.00%> (+0.05%)	⬆️
R/rct.R	100.00% <100.00%> (ø)
R/wk-crs.R	100.00% <100.00%> (ø)
R/wk-rcrd.R	100.00% <100.00%> (ø)
R/wk-vctr.R	100.00% <100.00%> (ø)
R/wkb.R	100.00% <100.00%> (ø)
R/wkt.R	100.00% <100.00%> (ø)

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