Big cleanup

[sonmarcho]

This PR performs the following modifications:

• we removed the region type parameter from the Ty type, which was used to make a distinction between types with regions and types with erased regions. This proved extremely cumbersome in the end: we sometimes had to realize conversions between the various kinds of types, and this prevented us from correctly using visitors in OCaml because of type-checking issues. In particular, this allowed us to derive more visitors (and more powerful) for the types defined in Charon-ML.
• we do not compute the regions hierachy (file regions_hierarchy.rs) in Charon anymore, and moved this analysis to Aeneas
• we updated the Name type so that the Impl blocks contain generic parameters and a type, closely following what happens in Rust. This allows a more general (and better) generation of names in Aeneas.
• we updated the printing functions in OCaml to make them a lot simpler
• we implemented a simple syntax for name patterns in Charon-ML, which allows to easily catch definitions that are to be mapped to definitions in the standard library. Those names look like: core::slice::{[@T]}::len (the len function defined in the impl block for type [T], where T is a type variable, in module core::slice), or core::ops::index::IndexMut<alloc::vec::Vec<@T, @A>, @T> (the instance of the trait core::ops::index::IndexMut for Vec<T, A> and T). We implemented a lot of utilities, which include: parsing, matching, conversion from names to patterns (which match those names), printing, functions to check if two patterns are convertible and to compute common prefixes, pattern maps (which map patterns to values and allow efficient lookups)
• in Charon-ML, we also added prefixes to the names of the enumeration variants so that they are less ambiguous and we don't need to use module names anymore to help OCaml typecheck (for instance, in type ty: Ref -> TRef)