nostrdb: add initial swift integration

nostrdb/AsciiCharacter.swift

@@ -0,0 +1,35 @@
+//
+//  AsciiCharacter.swift
+//  damus
+//
+//  Created by William Casarin on 2023-07-21.
+//
+
+import Foundation
+
+struct AsciiCharacter: ExpressibleByStringLiteral {
+    private let value: UInt8
+
+    var cchar: CChar {
+        return CChar(bitPattern: value)
+    }
+
+    init?(_ character: Character) {
+        guard let asciiValue = character.asciiValue, asciiValue < 128 else {
+            return nil
+        }
+        self.value = asciiValue
+    }
+
+    // MARK: - ExpressibleByStringLiteral conformance
+    init(stringLiteral value: StringLiteralType) {
+        guard value.count == 1, let character = value.first, let ascii = AsciiCharacter(character) else {
+            fatalError("Invalid ASCII character initialization.")
+        }
+        self = ascii
+    }
+
+    var character: Character {
+        return Character(UnicodeScalar(value))
+    }
+}