Switch to readr for everything

[ethanwhite]

1. We already operate primarily in the tidyverse
2. readr produces tibbles which have better display properties for displaying and automating output solution generation (see also #988)
3. We have to introduce readr for more complicated data loads anyway

• [x] Add readr all R-based installation instructions after R & RStudio
• [x] Install readr in all class workspaces
• [x] Rewrite solutions to use read_csv
• [x] Update all output solutions
• [x] Update all lecture notes
• [x] Update all exercises with import code

[ethanwhite]

Done! 5ae519e7e64ae12cbb47e9004c82f281fd6ee174