Add a concept exercise for Interfacing with C: structs

[oxe-i]

The first task focus on a float being passed in an integer register, and also on simple bit manipulations to fit everything in 2 registers.

The second task focus on a simple case of a difference in alignment between struct's fields, and the need to account for padding bytes.

The third task focus on a function returning a struct in memory and also on handling a slightly more complex case of misalignment within struct's fields.

The fourth task focus on managing an array of structs and also on returning a bool (which wasn't tested on the previous exercise).

The last task focus on passing a struct in memory as argument to a function.

[kotp]

Generally looking good. Some suggestions, though.