Make struct constructors PascalCase

[omegametabroccolo]

This PR implements #537 and fixes uses of struct constructors in lib/std and test. Not everything was covered by tests so I had to inspect the output of grep -R '[A-Z][a-z]*-[a-z]+' manually, I hope I didn't miss anything. Not covered:

• type constructors that don't follow this convention
• lib/v1

[TimWhiting]

@daanx

I think this is a useful and important change even though it breaks current code.
It matches Koka's style for many existing multiword type constructors.
In my code I've made all structs types for now and forced this convention.

It is better to make this breaking change now while the community is small and almost entirely on the community discord (judging by the Koka code I've seen on github and the number of users in the discord).