__log using new encode/decode

[xunilrj]

This issue is part of the new encode/decode scheme. See #4769.

To implement the new encoding for __log we need:

• to have a standard trait that all types can implement. We are going to call it AbiEncode. This trait will live inside the core module.
• to implement the functions and structs necessary to support the generated code. For example:

__log(1)

will be desugared into something like this:

let mut buffer = CodecBuffer::new();
encode(1, &mut buffer);
__log(buffer);

All this will live inside core::codec.
We may allow customization of the desugaring with attributes like rustc does with the panic handler.

• to implement AbiEncode for basic types such as integers, Vec, String etc...
• to support auto impl in the sense that if all Struct fields are AbiEncode, the compiler will auto-magically implement the obvious impl that will call encode on each field following declaration order;
  • See __log outputting encoded values #5306
  • Decide what to do with structs with type arguments
  • auto impl for enum, tuples etc...
• to implement the desugaring itself for the __log intrinsics;
• put the new encoding behing --experimental-codec
• to implement the rust codec crate that will be used by whoever needs it (Rust SDK etc...)