Adding type support for mutable vs. immutable slices

[tahina-pro]

tl;dr Breaking changes

If you need mutable slices, you will need to change your code to use Pulse.Lib.MutableSlice instead of Pulse.Lib.Slice.

Adding type support for mutable vs. immutable slices

With FStarLang/karamel#533, Karamel is on track to supporting F* interfaces implemented in Rust. Thus, mutability analysis requires annotations on the types of such interfaces, to determine which arguments should be mut and which shouldn't. (@msprotz, please correct me if I'm wrong.)

Following a suggestion by @gebner , this PR:

• makes Pulse.Lib.Slice.slice immutable
• introduces Pulse.Lib.MutableSlice.slice for mutable slices (although this wouldn't preclude Karamel's whole-program analysis from refining them as immutable.)

To implement immutable slices, I duplicated Pulse.Lib.ArrayPtr as Pulse.Lib.ConstArrayPtr for const array pointers, which I extract to C const pointers. To do so, I rely on type abstraction: supported operations are the same except op_Array_Assignment and copy, which I removed from Pulse.Lib.ConstArrayPtr.

For compatibility purposes, I left interfaces unchanged as much as possible. So, there is a lot of code duplication. Maybe we want a type class for slice operations? I don't know.

I need to:

• open a PR into Karamel to fix the MiniRust extraction of Pulse.Lib.Slice to mark them immutable, and add support for Pulse.Lib.MutableSlice.