avoid allocation in ObjectId formatter

Summary:
We currently use multiple string formatting approaches across the project. This diff stacks works at moving the project away from using `iostream`/`operator<<()`, `folly::format`, and `folly::to<std::string>`  and instead opting using the `fmt` library.

A few different things are mixed in with this stack, a diff will do some combination of the following:
1) removing definitions of `operator<<()` to discourage their use
2) removing definitions of `toAppend()` to discourage the use of `folly::to<std::string>` (though note, some diffs add this as a stepping stone to `fmt`
3) adding custom `fmt::formatter`s
4) adding unit tests for new and existing `fmt::formatter`s
5) cleaning up/optimizing existing `fmt::formatter`s

Reviewed By: taidaii

Differential Revision: D80026497

fbshipit-source-id: 98cf181cf77469c93404c05183a222515c1c8436

Author: genevievehelsel

eden/fs/model/ObjectId.h

@@ -219,10 +219,21 @@ struct hash<facebook::eden::ObjectId> {
 } // namespace std
 
 template <>
-struct fmt::formatter<facebook::eden::ObjectId> : formatter<std::string> {
+struct fmt::formatter<facebook::eden::ObjectId> {
+  constexpr auto parse(fmt::format_parse_context& ctx) {
+    return ctx.begin();
+  }
+
   template <typename Context>
   auto format(const facebook::eden::ObjectId& id, Context& ctx) const {
-    // TODO: avoid allocation here
-    return formatter<std::string>::format(id.toLogString(), ctx);
+    auto out = ctx.out();
+    constexpr char hexValues[] = "0123456789abcdef";
+
+    auto bytes = id.getBytes();
+    for (unsigned char b : bytes) {
+      *out++ = hexValues[b >> 4];
+      *out++ = hexValues[b & 0x0f];
+    }
+    return out;
   }
 };