feat: return Enums containing heap types at depth 1

[iqdecay]

This PR enables returning heap types surrounded in an Enum type. It notably enables returning
a Result<Vec> for instance from a contract call.

Closes #940.

Main disavantages from this approach:

• We are injecting more custom bytecode
• The interdependencies of the decoder and the bytecode make the code hard to navigate
• Enum can contain only one variant that uses a heap type inside. It could be extended to having an arbitrary number of them, but they would all have to have the same encoding width. This is a personal design choice, open to criticism.