Add optional operator labels for more diagnosable error messages (#742)

* Add optional operator labels for more diagnosable error messages

This adds an internal 'label' property to the operators that are
created as a graph is constructed, which MAY (in the RFC 2119 sense)
be used by implementations in async error messages. Developers
populate this via a 'label' member in the options dictionary for
MLGraphBuilder methods.

A new MLOperatorOptions dictionary is defined, and all existing
options dictionaries now inherit from this, and all relevant methods
now take an options dictionary.

Fixes #585

* Add note encouraging implementations

* Revise note to mention sync errors

* Update index.bs

Co-authored-by: Dwayne Robinson <[email protected]>

* don't pass options twice

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Co-authored-by: Dwayne Robinson <[email protected]>