Improve Snippet Trait

[aszepieniec]

The current Snippet trait contains redundant methods.

• Merge outputs and output_types so that the return type is Vec<(String, DataType)>
• Merge inputs and input_types so that the return type is Vec<(String, DataType)>
• Infer stack_diff from the previous.
• Update all existing snippets to this new trait interface.

Also, public_input and secret_input are not mutable vectors. As a result, you cannot string together multiple programs, each of which consume a part of public or secret input.

• Make public_input and secret_input mutable, in rust_shadowing.

[aszepieniec]

Also: add a new argument output, which is also a mutable vector.