[Backtracing] Add ImageMap instead of just using an Array.

We want to be able to efficiently serialise lists of images, and to do so
it makes most sense to create a separate `ImageMap` type.  This also provides
a useful place to put methods to e.g. find an image by address or by build
ID.

rdar://124913332

stdlib/public/libexec/swift-backtrace/TargetMacOS.swift

@@ -69,7 +69,7 @@ class Target {
   var crashingThread: TargetThread.ThreadID
 
   var task: task_t
-  var images: [Backtrace.Image] = []
+  var images: ImageMap
 
   var threads: [TargetThread] = []
   var crashingThreadNdx: Int = -1
@@ -193,7 +193,7 @@ class Target {
 
     mcontext = mctx
 
-    images = Backtrace.captureImages(for: task)
+    images = ImageMap.capture(for: task)
 
     fetchThreads(limit: limit, top: top, cache: cache, symbolicate: symbolicate)
   }