Migrate plyr::llply to lapply()

[MichaelChirico]

This migration is more straightforward -- llply() is basically a drop-in extension to lapply() that's equipped with some arguments to show progress, which is now dropped.

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.89%. Comparing base (74db873) to head (58b131c).
Report is 28 commits behind head on develop.

Files with missing lines	Patch %	Lines
R/var.R	75.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #136      +/-   ##
===========================================
+ Coverage    86.18%   86.89%   +0.71%     
===========================================
  Files           33       34       +1     
  Lines         3590     3982     +392     
===========================================
+ Hits          3094     3460     +366     
- Misses         496      522      +26     

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