[WX-1460] WDL 1.1 Struct Literal Parsing

[THWiseman]

This PR includes the majority of the work required to implement the Struct Literal Syntax of WDL 1.1 . At a high level, we can now parse the new syntax, and turn that into a value that can be validated & used.

This PR does not include the strict type checking defined in the spec. Since that will require some small tweaks to the linking step (and for the sake of the reviewers), I figure it's better to do that in a subsequent PR. In practice, this means that errors aren't thrown in certain situations where you would expect them to be thrown.

Examples:

struct MyStructType {
 Int myIntMember
 String? myStringMember
}

// the new syntax
MyStructType a = MyStructType{myIntMember: 4, myStringMember: "Hi!"}

// still works just like any old struct value
MyStructType b = a

// note that omission of optional members is allowed by the spec
MyStructType c = MyStructType{myIntMember: 3}

// Literals are values
Int someInt = (MyStructType{myIntMember: 5, myStringType: "Bye!"}).myIntMember

// As of this PR, this will not throw an error (but it technically should)
Boolean illegalBool = (MyStructType{myIntMember: 5, myStringType: "Bye!", iMadeUpThisBool: false}).iMadeUpThisBool