Unloading the module leaks a RunspacePool

[oising]

Due to #34 , unloading the module with remove-module polaris will leave the global variable $polaris and associated disposable RunspacePool field in memory. The Polaris class would be better modeled as a singleton, and be made disposable. Consider adding a hook for module removal to dispose the singleton; here's an interactive example:

PS C:\> new-module -Name foo {
    $m = $ExecutionContext.SessionState.Module;
    $m.OnRemove = {
        # [polaris]::cleanup()
        write-host "bye from $($m.Name)"
        }.GetNewClosure()
} | import-module

PS> remove-module foo
bye from foo