Remove pointers from the Sway IR.

[otrho]

Argh, this is hard

This is the result of several attempts (made over several weeks) to attack #2819 and is only the very first stage.

• Initially @vaivaswatha introduced a PR which removed Aggregate and extract_value/insert_value, replacing them with proper pointer types and GEP. This was great, but was too hard to make work with the ASM generation. There are too many assumptions around reference types vs copy types and what is a 'pointer' or not for a change like this to go in yet.
• I then tried paring it back to only introduce proper pointer types, leaving GEP for later, but still making it work with ASM gen (especially contract calls and storage operations) was too hard.

The problem

get_ptr was a bit nasty, as we use it to get the address of locals, but also to cast them to different types and also to index, a bit like GEP will.

Also, using Pointer for ref mut function args was a mistake as they were designed only for local variables.

Solution

This PR does as little as possible. The baby steps approach is absolutely required for this task and this is the first one.

To remove any ambiguity around how pointers should work this PR removes them altogether from IR, as they were actually not strictly doing anything. In particular it removes Pointer from Type and get_ptr is now get_local which is actually what 'pointers' were.

We will re-introduce pointers as actual types in an upcoming PR.

So now we have get_local which does nothing more than get a local variable address. Casting and indexing has been shunted to a new cast_ptr instruction as a stopgap. When we introduce GEP we can remove cast_ptr, or at least formalise it into a better/general cast.

This PR also uses the newer quad storage operations which support multiple slots, so the get_ptr -style indexing isn't actually required AFAICT.

Future stages

• Next we can convert Type into an arena and remove Aggregate. This is another ambiguity around 'reference types' which needs to be removed. The IR should not have any concept of copy types or reference types. This is a Sway thing and isn't portable to different back-ends.
• Then we can introduce Pointer as an actual Type. This will let us be explicit and strict about memory accesses and genuinely remove the notion of 'reference types'. We should consider opaque pointers as an alternative to pointer casting, which is required for dealing with storage slots and arrays of b256s.
• Then we replace extract_value and insert_value with GEP.
• Also work out how casting will work.

[vaivaswatha]

The change looks good to me. I didn't understand the change to fn compile_union_or_string_storage_(read/write) though. Hopefully the 2nd reviewer can check that.

[mohammadfawaz]

Awesome. The direction of this seems very reasonable to me.