Cache inferred types for sub expression

[hagl]

When importing an expression, the imported expression needs to be type-checked before it is included into the importing expression. When this expression is imported itself it will also be type-checked and in that process the first expression will be typechecked again.

Consider the following scenario

  A.dhall  --> B.dhall --> C.dhall       (where x --> y stands for x imports y)

The resolution phase will do the following steps:

Aᵣ = resolve A.dhall
  B = import B.dhall
  Bᵣ = resolve B.dhall
    C = import C.dhall
    Cᵣ = resolve C
    type-check Cᵣ
  type check Bᵣ (which now includes Cᵣ)
...

This gets worse the deeper the nesting of imports is.
A file might also be included multiple times through different paths. While the imported & type-checked expression is cached during one resolution phase, the included expression will still be type-checked again as part of the including expression.

For example tests/type-inference/success/preludeA.dhall includes around 100 files (all of prelude). And especially the more complex expression in the JSON, YAML and XML folders include a lot of the basic expressions from String, List and Map folders over and over again.

This causes the type-checking of this file to take multiple hours at the moment.

Storing the resolved type of a sub-expression inside the AST should greatly optimize this