Do not show coercion suggestion when coercion isn't possible

[zth]

Prior to this PR, this code:

let x: dict<JSON.t> = dict{"1": 1.}

...would give this error:

This has type: dict<float>
  But it's expected to have type: dict<JSON.t>
  
  The incompatible parts:
    float vs JSON.t (defined as JSON.t)
  
  Possible solutions: 
  - These types are compatible at runtime. You can use the coercion operator to convert to the expected type: (dict{"a": 1.} :> JSON.t)

Notice how it's suggesting a coercion that won't compile. That's because I was checking if the inner most types were coercable, but we must check the outer most types since it's the full type + expression that would need to be coerced.

This PR fixes that oversight.

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commit: 11fdc2f