use proper `conf-*` packages instead of depexts

[redianthus]

for instance conf-clang instead of the clang depext

[krtab]

The only ones that can currently be done seems to be clang, and maybe llvm, if using version 16 is fine. Both lld and wabt still have to be explicit depexts.

Depexts to change to conf-

• [x] clang (#205 )
• [ ] llvm (#206 ) -> Must split the llvm dependency as both llvm and opt.
• [ ] lld
• [x] wabt

[redianthus]

We could create the missing conf- packages for opam.

[filipeom]

Also, if we could adapt the reference interpreter's binary decoder to owi we wouldn't need wabt https://github.com/WebAssembly/spec/blob/main/interpreter/binary/decode.ml (but this is probably to much work)

[redianthus]

Yes, this is #9