refactor: Rename ArrowNestedType to ArrowTypeStruct for clarity

Author: Marco

Arrow/Sources/Arrow/ArrowArrayBuilder.swift

@@ -387,6 +387,7 @@ public class ArrowArrayBuilders {
 
     public static func loadTimestampArrayBuilder(_ unit: ArrowTimestampUnit, timezone: String? = nil) throws -> TimestampArrayBuilder {
         return try TimestampArrayBuilder(unit, timezone: timezone)
+    }
 
     public static func loadStructArrayBuilder(_ fields: [ArrowField]) throws -> StructArrayBuilder {
         return try StructArrayBuilder(fields)

Arrow/Sources/Arrow/ArrowType.swift

@@ -165,7 +165,7 @@ public class ArrowTypeTimestamp: ArrowType {
     }
 }
 
-public class ArrowNestedType: ArrowType {
+public class ArrowTypeStruct: ArrowType {
     let fields: [ArrowField]
     public init(_ info: ArrowType.Info, fields: [ArrowField]) {
         self.fields = fields