Add support for recursive messages (#130)

Changes message initialization (`__post_init__`) so that default values
are no longer eagerly created to prevent infinite recursion when
initializing recursive messages.

As a result, `PLACEHOLDER` will be present in the message for any
uninitialized fields.  So, an implementation of `__get_attribute__` is
added that checks for `PLACEHOLDER` and lazily creates and stores
default field values.

And, because `PLACEHOLDER` values don't compare equal with zero values,
a custom implementation of `__eq__` is provided, and the code generation
template is updated so that messages generate with `@dataclass(eq=False)`.

Also add new Message __repr__ implementation that skips PLACEHOLDER 
values and orders keys by number from the proto.

Co-authored-by: Christopher Chambers <chris@peanutcode.com>
Co-authored-by: nat <n@natn.me>
Co-authored-by: James <50501825+Gobot1234@users.noreply.github.com>

tests/inputs/recursivemessage/recursivemessage.json

@@ -0,0 +1,12 @@
+{
+  "name": "Zues",
+  "child": {
+    "name": "Hercules"
+  },
+  "intermediate": {
+    "child": {
+      "name": "Douglas Adams"
+    },
+    "number": 42
+  }
+}

tests/inputs/recursivemessage/recursivemessage.proto

@@ -0,0 +1,13 @@
+syntax = "proto3";
+
+message Test {
+    string name = 1;
+    Test child = 2;
+    Intermediate intermediate = 3;
+}
+
+
+message Intermediate {
+    int32 number = 1;
+    Test child = 2;
+}