Extend shank instructions to work with Instruction struct variants

[ngundotra]

This PR adds two features to ShankInstructions:

1. Parses Instruction variants with named arguments
2. Parses Instruction variants with multiple unnamed arguments (with names instructionArgs0, instructionArgs1, etc)

This is necessary to use Shank with SPL instructions. Closes #33

Example: Instruction variant with named arguments

Instructions with named arguments now works. The following

#[derive(ShankInstruction)]
pub enum Instruction {
    #[account(0, name = "creator", sig)]
    #[account(1, name = "thing", mut)]
    CreateThing {
        some_args: SomeArgs,
        other_args: OtherArgs,
    },
    #[account(0, name = "creator", sig)]
    CloseThing(Option<u8>),
}

compiles to:

{
  "version": "",
  "name": "",
  "instructions": [
    {
      "name": "CreateThing",
      "accounts": [
        {
          "name": "creator",
          "isMut": false,
          "isSigner": true
        },
        {
          "name": "thing",
          "isMut": true,
          "isSigner": false
        }
      ],
      "args": [
        {
          "name": "someArgs",
          "type": {
            "defined": "SomeArgs"
          }
        },
        {
          "name": "otherArgs",
          "type": {
            "defined": "OtherArgs"
          }
        }
      ],
      "discriminant": {
        "type": "u8",
        "value": 0
      }
    },
    {
      "name": "CloseThing",
      "accounts": [
        {
          "name": "creator",
          "isMut": false,
          "isSigner": true
        }
      ],
      "args": [
        {
          "name": "instructionArgs",
          "type": {
            "option": "u8"
          }
        }
      ],
      "discriminant": {
        "type": "u8",
        "value": 1
      }
    }
  ],
  "metadata": {
    "origin": "shank"
  }
}

Example: Instruction variant with multiple unnamed arguments

Previously this would throw with an error saying "An Instruction can only have one arg field". But now it works. The following

#[derive(ShankInstruction)]
pub enum Instruction {
    #[account(0, name = "creator", sig)]
    CloseThing(Option<u8>, ComplexArgs, ComplexArgs),
}

compiles to

{
  "version": "",
  "name": "",
  "instructions": [
    {
      "name": "CloseThing",
      "accounts": [
        {
          "name": "creator",
          "isMut": false,
          "isSigner": true
        }
      ],
      "args": [
        {
          "name": "instructionArgs0",
          "type": {
            "option": "u8"
          }
        },
        {
          "name": "instructionArgs1",
          "type": {
            "defined": "ComplexArgs"
          }
        },
        {
          "name": "instructionArgs2",
          "type": {
            "defined": "ComplexArgs"
          }
        }
      ],
      "discriminant": {
        "type": "u8",
        "value": 0
      }
    }
  ],
  "metadata": {
    "origin": "shank"
  }
}

Todo:

• Add change log

[thlorenz]

Very nice work, I reviewed and also ran the IDL against solita to see what the generated code looks like.
One improvement we could do in the future is to represent the instructions args as tuples, i.e. [u8, String, ComplexType] instead, but for now this solution is fine.

I fixed one minor nit (instructionArgs seemed a bit long to me, also when we're talking about one arg it shouldn't be named instructionArgs0, but instructionArg0 or just arg0).

Aside from that all checks out and I'm happy to merge if you're fine with my modification.

[ngundotra]

Absolutely! Go for it. Thanks Thorsten!