Fixes #503 - reintroduces a debug flag and prevents duplicate assembly in the AppDomain when not debugging

 - REPL - uses `RoslynScriptEngine` (the basic one)
 - `debug` flag - uses `RoslynScriptInMemoryEngine` (no debug for REPL)
 - `inMemory` flag - switch between `RoslynScriptEngine` (same as REPL) and `RoslynScriptPersistentEngine`

Debug overrides inMemory if both present.

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