Implementation of a [[literal-function]] constructor. Literal functions can be
applied to structures and numeric inputs, and differentiated.

The namespace also contains an implementation of a small language for
declaring the input and output types of [[literal-function]] instances.

Symbolic expressions in Emmy are created through the [[literal-number]]
constructor, or implicitly by performing arithmetic between symbols and
numbers.

This namespace implements the [[literal-number]] constructor and installs the
underlying type into the generic arithmetic system.