Feat: Cache StableMir results and basic compiler ctx

[zjp-CN]

This PR

• stores StableMir results and basic compiler contexts such as TyCtx and SourceMap in thread local variables
  • close Cache body or some StableMir results #33
• moves file field from SerFunction to SourceCode

[
  {
    "hash": "80162896300613540515264293433698156925",
    "def_id": "DefId { id: 52, name: \"verify::single_contract\" }",
    "attrs": [
      "#[kanitool::proof]"
    ],
    "func": {
      "file": "tests/proofs/standard_proofs_with_contracts.rs",
      "src": "fn single_contract() {\n        let val = contract(1);\n        assert!(val > 0);\n    }",
      "before_expansion": null
    },
    "callees": [
      {
        "def_id": "DefId { id: 271, name: \"kani::assert\" }",
        "func": {
          "file": "kani/library/kani_core/src/lib.rs",
          "src": "pub const fn assert(cond: bool, msg: &'static str) {\n            let _ = cond;\n            let _ = msg;\n        }",
          "before_expansion": "kani_core::kani_intrinsics!(std)"
        }
      },
...