Rewrite serialization code to avoid JSON

Breaking change: automereg will no longer respect `json:` tags on your
structs, instead it uses automerge: tags.

This also fixes a variety of bugs that would happen when certain types
were not the top-level type for automerge.As[]

automerge.go

@@ -69,6 +69,23 @@ assume the path is of the type you say it is:
 When you do this, any errors caused by traversing the path will be returned from
 methods called on the returned objects.
 
+# Controling formatting of structs
+
+By default automerge will convert your struct to a map. For each public field in the
+struct (the name starts with an uppercase letter) automerge will add an entry to the
+map with the name of the field and the fields value converted to an automerge value
+recursively.
+
+You can control this behaviour using struct tags:
+
+	struct Example {
+		Renamed bool `automerge:"newname"`
+		Private bool `automerge:"-"`
+	}
+
+If the tag is present and equal to "-" the field will be ignored by automerge,
+otherwise the fields name will be set to the value of the tag.
+
 # Syncing and concurrency
 
 You can access methods on [*Doc] from multiple goroutines and access is mediated

example_test.go

@@ -24,7 +24,7 @@ func ExampleAs() {
 	fmt.Println("foo-bar:", v)
 
 	type S struct {
-		IsValid bool `json:"isValid"`
+		IsValid bool `automerge:"isValid"`
 	}
 	s, err := automerge.As[*S](doc.Root())
 	if err != nil {