Provide some more map helpers like the C++ Descriptor (take 2).

Migrate some generator code over to the descriptors also since it makes
the sub fields a little less black magic.

Author: thomasvl

Sources/SwiftProtobufPluginLibrary/Descriptor.swift

@@ -159,8 +159,6 @@ public final class Descriptor {
   public private(set) weak var file: FileDescriptor!
   public private(set) weak var containingType: Descriptor?
 
-  public let isMapEntry: Bool
-
   public let enums: [EnumDescriptor]
   public let messages: [Descriptor]
   public let fields: [FieldDescriptor]
@@ -179,6 +177,17 @@ public final class Descriptor {
     return proto.extensionRange
   }
 
+  /// True/False if this Message is just for a `map<>` entry.
+  public var isMapEntry: Bool { return proto.options.mapEntry }
+
+  /// Returns the `FieldDescriptor`s for the "key" and "value" fields. If
+  /// this isn't a map entry field, returns nil.
+  public var mapKeyAndValue: (key: FieldDescriptor, value: FieldDescriptor)? {
+    guard isMapEntry else { return nil }
+    assert(fields.count == 2)
+    return (key: fields[0], value: fields[1])
+  }
+
   public var useMessageSetWireFormat: Bool { return proto.options.messageSetWireFormat }
 
   fileprivate init(proto: Google_Protobuf_DescriptorProto,
@@ -190,8 +199,6 @@ public final class Descriptor {
     let fullName = "\(prefix).\(proto.name)"
     self.fullName = fullName
 
-    isMapEntry = proto.options.mapEntry
-
     self.enums = proto.enumType.enumeratedMap {
       return EnumDescriptor(proto: $1, index: $0, registry: registry, fullNamePrefix: fullName)
     }

Sources/protoc-gen-swift/Descriptor+Extensions.swift

@@ -96,11 +96,8 @@ extension Descriptor {
 
 extension FieldDescriptor {
   func swiftType(namer: SwiftProtobufNamer) -> String {
-    if isMap {
-      let mapDescriptor: Descriptor = messageType
-      let keyField = mapDescriptor.fields[0]
+    if case let (keyField, valueField)? = messageType?.mapKeyAndValue {
       let keyType = keyField.swiftType(namer: namer)
-      let valueField = mapDescriptor.fields[1]
       let valueType = valueField.swiftType(namer: namer)
       return "Dictionary<" + keyType + "," + valueType + ">"
     }
@@ -227,11 +224,8 @@ extension FieldDescriptor {
   /// Calculates the traits type used for maps and extensions, they
   /// are used in decoding and visiting.
   func traitsType(namer: SwiftProtobufNamer) -> String {
-    if isMap {
-      let mapDescriptor: Descriptor = messageType
-      let keyField = mapDescriptor.fields[0]
+    if case let (keyField, valueField)? = messageType?.mapKeyAndValue {
       let keyTraits = keyField.traitsType(namer: namer)
-      let valueField = mapDescriptor.fields[1]
       let valueTraits = valueField.traitsType(namer: namer)
       switch valueField.type {
       case .message:  // Map's can't have a group as the value