op2++: codegen for &self parameter

[littledivy]

Refactor out codegen for &self from #682

struct State {
  name: &'static str,
}

impl State {
  #[op2(fast, method(State)]
  fn print(&self) {
    println!("{}", self.name);
  }
}

const STATE_DECL: [OpDecl; 1] = [
  State::print()
];

[littledivy]

Function callback arguments are needed for: args.this()

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.25490% with 13 lines in your changes are missing coverage. Please review.

Project coverage is 81.29%. Comparing base (5245f5d) to head (4f95660).

Files	Patch %	Lines
testing/checkin/runner/ops.rs	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #686      +/-   ##
==========================================
+ Coverage   81.26%   81.29%   +0.02%     
==========================================
  Files          95       95              
  Lines       24295    24385      +90     
==========================================
+ Hits        19744    19823      +79     
- Misses       4551     4562      +11     

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[mmastrac]

LGTM!