The List and Dict types are no longer implicitly available. They must be
imported from typing.
Type annotations must appear before the doc comment in a function. Also
fix type errors in these functions that weren't detected before.
A few operands have a fixed type assigned. Create a singleton type
variable for these exceptions. Most instructions are polymorphic, so
this is a little overhead.
Eliminate the Operand.typ field and replace it with an Operand.typevar
field which is always a TypeVar, but which only exists in VALUE
operands.
