fix debugging channels

[ghost]

• Remove unnecessary "0" argument to make chan
• Do not close the debugging channels when the VM completes (for vm: Add basic support for VM instances to be reused (via Reset()) #59)
• Change VM.Step() to return a channel. This allows the step channel to be
  used from inside a select, so the operation can be aborted by the caller
• Change VM.Position() to return a read-only channel