Disallow --target-dir in ChiselStage$ (#3063)

* Refactor Shell into BareShell and Shell

Refactor firrtl.options.Shell into BareShell and Shell.  This is done to
enable the leaner BareShell to _not_ provide no command line options
other than "--help".  This enables use cases like the ChiselStage$
object which should not have options that are available to a Shell,
e.g., "--target-directory".

* Use BareShell in ChiselStage$ options (remove -td)

Change the ChiselStage$ object to use BareShell which enables this to
now reject options like "--target-directory"/"-td" which have no
effect.  Add a test that "-td" is rejected by emitCHIRRTL.

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Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>