Improvements & Extensions

ValueStore

• add convenience methods when dealing with ValueStore of Collections
• make update to be able to forward returned value
• improve ValueStore threading access model

Misc

• add CGImage setialization to/from data and file
• conform SpellbookLogLevel to CustomStringConvetible
• improve safety of Array[safe:]

Filesystem

• add FileManager.fileExists(atPath path: String, isDirectory: inout Bool) -> Bool
• add URL.ensureFileExists()