Spawner in Env

[magicant]

Consider holding a local spawner in Env instead of in VirtualSystem's state. It might be useful even with RealSystem when implementing here-document and virtual subshell, for example.

• Move Executor from yash_env::system::virtual to yash_env
• Move executor from SystemState to Env
• Remove executor from DummyChildProcess

[magicant]

I'm not sure if this idea is worth doing, so I give it a low priority now.

• Implementing here-documents with virtual child processes that write to a pipe would require blocking SIGPIPE and complicate signal handling. I'd rather implement them with a temporary file.
• implementing virtual subshells requires much consideration. It is not yet very clear if having a spawner in Env contributes to a better design.