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`.
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