As discussed in #311, I've added a test program to check that the crate is usable without alloc.

This is pretty belt-and-braces - there's two sections to this.

1. The workflow tests compilation with the various possible combinations of features
2. I've added a "no_alloc_check" executable along the lines of what I wrote in PhysicalMapping cannot be dereferenced. #311. This really is a check that the crate doesn't expose any linker requirement on alloc. I'm not sure that Rust would even allow that, but we might as well check.