Add support for typecasting in mapping generation on cli init

[travs]

This issue has been raised before (#949, #1017), but when generating a new subgraph for a contract that has events with address[] inputs, the generated mappings won't compile properly.

This is due to an issue where arrays of derived types (Address[] in this case) are not assignable to arrays of base types (Bytes[] here).

I wasn't sure how to fix this on a lower level as I'm not familiar with AssemblyScript, so I went and implemented a fix where the mapping assignments generated for events on init are conditionally wrapped with changetype<typeB>(...parameter.of.typeA...) if typeA is not able to be implicitly cast to typeB.

This got a bit messy and I had to extend some functions since we also must add imports for the types we are typecasting as (e.g. Bytes), and these must be calculated before we iterate events inside the template string.

I tested this locally and the generated mappings compile, but I'm not too sure about events with tuples, since they were a special case to begin with. A solution that doesn't do typecasting, but improves the base types or better uses AssemblyScript's existing type system is probably superior to my approach.

[changeset-bot]

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