Commit ffe45ecce had introduced a regression with some
polymorphic functions imported between typed modules due to
miscommunicated variance information.
Split out base-abbrev.rkt so that subtype is not dependent on abbrev.rkt.
Remove unused code in numeric-tower.rkt so that it is now a dependent of
abbrev.rkt, which allows the body of convenience.rkt to be merged back in.
Remove special casing for union.rkt and extraneous subtyping checks.
Remove union-maker.
Previously, some hacks were used to obtain the internal
identifiers that implemented keyword functions directly, and
give them types at startup. Now, the primary "function"
(eg, `sort`) is given a type, and when used, the residual
syntax properties are used to find `sort` from the real
functions, and then the type of the real function is computed
from the type of `sort`.
Some creativity was required in the types of functions which
take optional arguments that when present, alter the return type,
such as `regexp-match*` and `file->list`.
