Add a suite of unicode math characters

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(See also #5.)

Add support for different Unicode characters that can be easily used. Wikipedia has a nice list of these symbols.

Formulador currently has some preliminary support for Unicode characters, but they're piecemeal. See charmap.lisp and constructions.lisp.

For example, suppose we want to support arrows. Then maybe we create a file called unicode/arrows.lisp and allow different ways for painting different types of arrows, from usual ASCII |--> arrows to nice Unicode ⟼ arrows.