Remove PROCESS_INFORMATION from Execution

[iCharlesHu]

#93 added PROCESS_INFORMATION as a property to Execution, making it no longer "stateless". We want to eventually make Execution ~Copyable so we should remove PROCESS_INFORMATION from Execution and find another way to save the process handle.

[jakepetroules]

Why is the presence of PROCESS_INFORMATION incompatible with Execution being ~Copyable?

[compnerd]

I think that adding a retroactive Sendable conformance would be the right thing to do. As @jakepetroules mentioned, PROCESS_INFORMATION shouldn't be incompatible with ~Copyable.