Update UMP.agda, NaturalModels/, Syntax/ to build again

[hejohns]

In 325c84b, files under NaturalModels, Syntax, and UMP.agda were removed because they didn't build anymore.

Fix-up UMP.agda and the other files.

#90 (comment)

[hejohns]

For convenience, I've attached the git revert patch to 325c84b.
Although, owing to

Note: If you use Linux and try to upload a .patch file, you will receive an error message. This is a known issue.

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/attaching-files

I've attached it as a .md. It's really a git format-patch -n HEAD^1.
0001-Revert-remove-everything-that-doesn-t-build-and-make.md

[maxsnew]

I'm working on reformulating some of this stuff to use elimination principles etc to see if we can more directly apply our gluing techniques to lambda calculi viewed as free simple categories with families rather than free CCCs