Remove unnecessary uses of Show and PrettyPrint

 * Use datatypes for evaluation instead of typeclasses
 * Add comments

And fix some hlint errors

This reverts commit cd4d408f.

Data.Semigroup

Add an `etaConvert` function. However, its not referenced because its
results can be confusing in certain situations

 * Reword STLC/syntax secction
 * Add typing examples

Fix formatting of context section

Reword introduction

Add typechecking rules for System F

 * Restructure headings
 * Add type abstraction
 * Add syntax examples

 * Add syntax section
 * Remove trailing whitespace

Add a limitations section for STLC

Add a typecheck section for STLC

 * Reword syntax paragraphs
 * Add an example