Use #arity to determine # of args for instructions

For this, I assumed that `args` is the number of arguments for the instruction in the following:

```ruby
instruction :mov, opcode: 0x02, args: 2, argsize: 2 do |src, dest|
  # ...
end
```

Assuming that is true, this PR makes it automatically determine the number of arguments, based on the arity of the block, so now only this is required:

```ruby
instruction :mov, opcode: 0x02, argsize: 2 do |src, dest|
  # ...
end
```