mStructs with encodingKey (integrated with source-mapping branch)

[rjharmon]

Helios up until v0.17.0.98 allowed definition of a string-mapped struct (aka mStruct) in this form

struct MyStructDef {
  name: String
  description: String  "descr"
  otherField: Int "ofld"  // one field needs an explicit encoding key defined
}

This was encoded as a ConstrData(0, MapData), and it worked fine when a script's Datum was itself defined as an mStruct such as this myStructDef, and not an enum with a variant having field1: mStruct. Using that definition inside an enum variant would produce a second layer of ConstrData.

This change establishes:

• Datum enum variants with field1 as an mStruct will match the CIP-68 requirements (the Enum's ConstrData element provides that aspect of the requirement)
• Use of an mStruct in any position does no longer have a ConstrData wrapper, because it would be redundant

This represents a behavioral change versus version 0.16 (which expects a ConstrData in all cases when decoding an mStruct). Developers using such a struct as their Datum can continue getting CIP-68-compliant encoding by changing to an enum:

enum Datum {
  ForCip68 {
    data: MyStructDef
    version: String
    extraData: <any type>
  }
}

[christianschmitz]

Thanks for this contribution, great work!

[rjharmon]

thanks for the guidance on the Expr bit