Data Structures used in Typechecking.
The main data structure used in typechecking is a tc-results/c.
This currently has two variants
(struct/c tc-results ((listof (struct/c tc-result (Type/c FilterSet? Object?)))
                      (or/c #f (cons/c Type/c symbol?))))
(struct/c tc-any-results (or/c Filter/c NoFilter?))

The first represents a fixed number of values with optional dotted return values.
The second represents an unknown number of values.

A value has three main parts: a Type, a FilterSet, and an Object. For dotted values we do no store a 
FilterSet or an Object because they would almost never be useful. They are thus implicitly -top-filter
and -empty-obj. In the tc-any-results case since we don't know the number of values, we do not store
the Type or the Object, but we do store a filter. This is useful in cases like
(let ((x (read)))
  (unless (number? x) (error 'bad-input))
  (do-stuff x))


Inference and Sequences
Typed racket has many types which can have a subpart which is a sequence of different types, for
example heterogenous lists or arguments to a function. We assume such sequences take one of three forms.
A fixed prefix followed by either a dotted tail, a uniform tail, or no more types. The subtyping
relationship has 9 different cases corresponding to the product of the cases. This is implemented once
for a generic sequence, and each other type converts itself to a sequence to implement inference.
