Add a typecheck section for STLC

 * Reword syntax paragraphs
 * Add an example

Add a basic typing rule for STLC

Start, instead, with the STLC

Move type context up

Add an introductory section for "Simple Types"

 * Complete the introduction
 * Add citations

uniques is now an infinite list of:

a..z, a0..z0, a1..z1, ...