idmap2: switch to improved Result class

Remove the old std::optional based Result class, replace uses with the
new std::variant based Result class.

Test: make idmap2_tests
Change-Id: I401cb36e5af06133a2872d835cf29bfb0b106597
diff --git a/cmds/idmap2/libidmap2/ZipFile.cpp b/cmds/idmap2/libidmap2/ZipFile.cpp
index 15ec3f9..0f07324 100644
--- a/cmds/idmap2/libidmap2/ZipFile.cpp
+++ b/cmds/idmap2/libidmap2/ZipFile.cpp
@@ -59,7 +59,10 @@
 Result<uint32_t> ZipFile::Crc(const std::string& entryPath) const {
   ::ZipEntry entry;
   int32_t status = ::FindEntry(handle_, ::ZipString(entryPath.c_str()), &entry);
-  return status == 0 ? Result<uint32_t>(entry.crc32) : kResultError;
+  if (status != 0) {
+    return Error("failed to find zip entry %s", entryPath.c_str());
+  }
+  return entry.crc32;
 }
 
 }  // namespace android::idmap2