Fall 2021 release

Frozen version from Fall 2021. Changes over last release are mostly typos, but also two minor modifications for definitions:

• We measure the size of a circuit or straightline program by the number of gates/lines. For example, the constant zero function on n inputs can be computed by a circuit of at most 10 gates, independently of n. This means that the class SIZE(100) for example contains infinitely many functions, but we typically try to only use SIZE_n(100) or SIZE_{n,m}(100) which are finite classes.

• The output of a Turing machine is the contents of the tape between the "begin tape" and the "empty" symbols. Previously it was until the first head position. The new convention makes composition easier, though notes it means a Turing machine can compute the identity function in constant time.