Avoid nested chained let expressions across Dafny code

[alex-chew]

The Dafny-to-Java compiler compiles nested chained-let expressions into Java code that causes javac to OOM easily: dafny-lang/dafny#3868. Unfortunately it seems that it will not be quick to fix the root cause in the Dafny-to-Java compiler, so to ensure this project's stability we should remove instances of this code pattern.

#117 removes all instances from the DynamoToStruct module, but we should search for similar hotspots elsewhere in the Dafny code and eliminate them.