Fix record cast with named record type

[bentsherman]

This PR fixes the use of casting records to a named record type, for example:

workflow {
    record(id: '1', fastq: file('1.fastq')) as Sample
}

record Sample {
    id: String
    fastq: Path
}

Casting a record to a named record type should be a no-op at runtime. It is only used by the type checker to validate that the record satisfies the requirements of the record type.

If the cast is is not removed at runtime, the record would be cast to an instance of Sample, since record types are compiled to actual classes at runtime with an implicit constructor. This would break the runtime's assumption that all records are RecordMaps.

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[jorgee]

In my opinion, this behaviour is strange or at least incomplete; a user could do something like this:

record Sample{
	id:String
	num: Integer
}

workflow{
	def a = record(id: "hello", num: 1) as Sample
	if (a instanceof Sample){
		println("It is a sample")
	}else{
		println("not a sample")
	}
}

You create a record, cast it to Sample and then check the type with instanceof and see that it is not really a Sample. Not sure, if it is possible to replace the instanceof by a method that checks the defined fields instead of the class. At least we should add a note describing it in the docs. (I haven't found anything about it)

[bentsherman]

Good catch. I actually meant to implement a compiler transform for that as well, so that sample instanceof Sample becomes something like _instanceof_record(sample, Sample) which performs a runtime check. I guess that fell through the cracks during development

I will try to implement that in this PR