feat(NIP-20): send command results to clients

When submitting events to relays, clients currently have no way to know
if an event was successfully committed to the database. This NIP
introduces the concept of command results which are like NOTICE's except
provide more information about if an event was accepted or rejected.

A command result is a JSON object with the following structure that is
returned when an event is successfully saved to the database or
rejected:

	["OK", <event_id>, <true|false>, <message>]

nip20: https://github.com/nostr-protocol/nips/pull/62

src/notice.rs

@@ -0,0 +1,86 @@
+pub enum EventResultStatus {
+    Saved,
+    Duplicate,
+    Invalid,
+    Blocked,
+    RateLimited,
+    Error,
+}
+
+pub struct EventResult {
+    pub id: String,
+    pub msg: String,
+    pub status: EventResultStatus,
+}
+
+pub enum Notice {
+    Message(String),
+    EventResult(EventResult),
+}
+
+impl EventResultStatus {
+    pub fn to_bool(&self) -> bool {
+        match self {
+            Self::Saved => true,
+            Self::Duplicate => true,
+            Self::Invalid => false,
+            Self::Blocked => false,
+            Self::RateLimited => false,
+            Self::Error => false,
+        }
+    }
+
+    pub fn prefix(&self) -> &'static str {
+        match self {
+            Self::Saved => "saved",
+            Self::Duplicate => "duplicate",
+            Self::Invalid => "invalid",
+            Self::Blocked => "blocked",
+            Self::RateLimited => "rate-limited",
+            Self::Error => "error",
+        }
+    }
+}
+
+impl Notice {
+    //pub fn err(err: error::Error, id: String) -> Notice {
+    //    Notice::err_msg(format!("{}", err), id)
+    //}
+
+    pub fn message(msg: String) -> Notice {
+        Notice::Message(msg)
+    }
+
+    fn prefixed(id: String, msg: &str, status: EventResultStatus) -> Notice {
+        let msg = format!("{}: {}", status.prefix(), msg);
+        Notice::EventResult(EventResult { id, msg, status })
+    }
+
+    pub fn invalid(id: String, msg: &str) -> Notice {
+        Notice::prefixed(id, msg, EventResultStatus::Invalid)
+    }
+
+    pub fn blocked(id: String, msg: &str) -> Notice {
+        Notice::prefixed(id, msg, EventResultStatus::Blocked)
+    }
+
+    pub fn rate_limited(id: String, msg: &str) -> Notice {
+        Notice::prefixed(id, msg, EventResultStatus::RateLimited)
+    }
+
+    pub fn duplicate(id: String) -> Notice {
+        Notice::prefixed(id, "", EventResultStatus::Duplicate)
+    }
+
+    pub fn error(id: String, msg: &str) -> Notice {
+        Notice::prefixed(id, msg, EventResultStatus::Error)
+    }
+
+    pub fn saved(id: String) -> Notice {
+        Notice::EventResult(EventResult {
+            id,
+            msg: "".into(),
+            status: EventResultStatus::Saved,
+        })
+    }
+}