Adopted the standard withCString for C strings throughout the library.

Sources/PlaydateKit/File/Classes/Handle.swift

@@ -9,7 +9,7 @@ extension File {
 
         /// Opens the file at `path`.
         public init(path: String, mode: Options) throws(PlaydateError) {
-            let pointer = path.withPlaydateCString {
+            let pointer = path.withCString {
                 fileAPI.pointee.open.unsafelyUnwrapped($0, mode.cValue)
             }
             guard let pointer else { throw lastFileError() }
@@ -71,8 +71,8 @@ extension File {
         /// Writes the string's UTF-8 to the file. Returns the bytes written.
         @discardableResult
         public func write(_ string: String) throws(PlaydateError) -> Int {
-            let result = string.withPlaydateUTF8 { bytes, count in
-                fileAPI.pointee.write.unsafelyUnwrapped(pointer, bytes, UInt32(count))
+            let result = string.withCString { cString in
+                fileAPI.pointee.write.unsafelyUnwrapped(pointer, cString, UInt32(string.utf8.count))
             }
             if result < 0 { throw lastFileError() }
             return Int(result)