- [x] downgrade stage
- [ ] resolve stage
  - [ ] dynamic attr resolution
  - [ ] import resolution
    - [ ] static path resolution
- [ ] eval stage
  - [ ] dynamic path resolution
  - [ ] graph update
  - [ ] stack storage

依赖类型：
1. 强依赖 (x!)
  - builtins.toString x => x!
  - x + y => x! y!
  - { x = 1; ${sym} = 2; } => sym!
2. 弱依赖 a.k.a. thunk (x?)
  - builtins.seq x y => x! y?
3. 递归依赖 (x!!)
  - builtins.deepSeq x y => x!! y?

e.g.
- let a? = { inherit a?; }; in a! => a!
  - a! => { a = <THUNK> }
- f: let x? = f! x?; in x! => f! x!
- let ret? = (self: { x? = 1; y? = self.x! + 1; }) ret?; in ret.y! => ret! x! y!

工作流程：
1. string -> AST
2. AST -> HIR (alloc ExprId)
3. HIR -> LIR (resolve var, build graph)
4. LIR -> Value