Convert BuildModule to use TVM node system (#879)

* Make python BuildConfig serializable/deserializable to/from string

* Make C++ BuildConfig serializable/deserializable to/from string

* Revert "Make python BuildConfig serializable/deserializable to/from string"

This reverts commit a5e1fb3ff63a161cc0d63475d2a32816cc4c3666.

* Revert "Make C++ BuildConfig serializable/deserializable to/from string"

This reverts commit ec0c2c54543050fe6f264d06eebff33dee70370b.

* Converted BuildConfig to use TVM node system

* Fix lint

* Fix lint

* Added code to set node attributes through the C API

* Fixed bug in build_config()

* Fix lint

* Fix lint

* Fix test errors

* Reduced scope of node __setattr__ to apply only to BuildConfig

* Fix lint

* Fix lint

* Changed python BuildConfig to be immutable, with values set once on construction.

* Fix lint

* Fix C++ test

* Fixed BuildConfig setting python-side args

* Fix lint

* Removed dependency on reflection.cc to construct BuildConfig (allow use in runtime library)

* Fix lint

* Revert "Fix lint"

This reverts commit 16ed6d7a1ca5e551b035bad46e8361ea487cd45b.

* Revert "Removed dependency on reflection.cc to construct BuildConfig (allow use in runtime library)"

This reverts commit 43817c97a2ee045791e0c031d962fa97636ce8f6.

* Avoid accessing BuildConfig when using runtime lib

* Fix missing import

* Fix error running under cython (root cause: node handle is not valid until after __init__ has returned, so cannot call __dir__ during __init__

* Fix error where BuildConfig._node_defaults was not copied in build_config()

* Fix lint

* Fix lint

* Fix lint

* Fix lint

* Add comments to python BuildConfig