feat(nostr): nostr-based account registration and verification

Rebuilt the nostr registration feature cleanly on top of current ergo
mainline (origin/master), dropping the stale ergo reversions that were
entangled in the original squashed branch.

- New irc/nostr package: NIP-04/NIP-17 DMs, NIP-05 resolution, relay
  connect/auth, and identifier helpers.
- Registration/verification via nostr: parseCallback auto-detects nostr
  identifiers (NIP-05, npub, hex pubkey) and dispatches a verification DM.
- Nostr-based cloaked hostnames for accounts (opt-in via
  ip-cloaking.nostr-hostnames), applied at the account-cloak sites.
- Config: accounts.registration.nostr-verification and the
  nostr-hostnames cloak option, with matching default.yaml and help text.
- Adds github.com/nbd-wtf/go-nostr and vendored dependencies.

vendor/github.com/json-iterator/go/iter_array.go

@@ -0,0 +1,64 @@
+package jsoniter
+
+// ReadArray read array element, tells if the array has more element to read.
+func (iter *Iterator) ReadArray() (ret bool) {
+	c := iter.nextToken()
+	switch c {
+	case 'n':
+		iter.skipThreeBytes('u', 'l', 'l')
+		return false // null
+	case '[':
+		c = iter.nextToken()
+		if c != ']' {
+			iter.unreadByte()
+			return true
+		}
+		return false
+	case ']':
+		return false
+	case ',':
+		return true
+	default:
+		iter.ReportError("ReadArray", "expect [ or , or ] or n, but found "+string([]byte{c}))
+		return
+	}
+}
+
+// ReadArrayCB read array with callback
+func (iter *Iterator) ReadArrayCB(callback func(*Iterator) bool) (ret bool) {
+	c := iter.nextToken()
+	if c == '[' {
+		if !iter.incrementDepth() {
+			return false
+		}
+		c = iter.nextToken()
+		if c != ']' {
+			iter.unreadByte()
+			if !callback(iter) {
+				iter.decrementDepth()
+				return false
+			}
+			c = iter.nextToken()
+			for c == ',' {
+				if !callback(iter) {
+					iter.decrementDepth()
+					return false
+				}
+				c = iter.nextToken()
+			}
+			if c != ']' {
+				iter.ReportError("ReadArrayCB", "expect ] in the end, but found "+string([]byte{c}))
+				iter.decrementDepth()
+				return false
+			}
+			return iter.decrementDepth()
+		}
+		return iter.decrementDepth()
+	}
+	if c == 'n' {
+		iter.skipThreeBytes('u', 'l', 'l')
+		return true // null
+	}
+	iter.ReportError("ReadArrayCB", "expect [ or n, but found "+string([]byte{c}))
+	return false
+}