add thunks concept and exercise

[oxe-i]

This adds a thunks concept and its exercise. This concept has recursion as a prerequisite (which in turn has stack as a prerequisite) and is meant as an introduction to functional patterns in assembly.

The exercise has a test file in C, as usual, but also a second .asm file with a global function, clobber, called from the test file. clobber takes a value and forwards it while also inserting -1 into all caller-saved registers (except rax). This is to force students to pay close attention to the correct use of caller and callee-saved registers when calling external functions.

I tried to make a shorter concept, with a more focused exercise, so I left the specifics of functional patterns for a possible future concept.

I used Claude (a LLM) to revise the text for the concept and for the exercise, and to generate test cases. It also suggested the theme for the exercise (bookkeeping), which seemed a good fit.