Instantiate refactor + support for instantiating dataclasses

[omry]

Taking over #1236 (cc @guillaumegenthial).

Closes #1183
Closes #1171
Closes #1410

Changes:

1. removed parse mode for now (It was not documented and not tested and it didn't seem to be needed for anything).
2. Refactor tests to allow testing multiple instantiate functions in parallel
3. Added a second instantiate function and simplified it (187 vs 240 lines).
4. I also added some more tests covering convert modes.
5. After taking a close look, I agree that it's very difficult to support passthrough of dataclass instances so for now I am officially unsupporting it. docs says that passthrough dataclasses or instances are interpreted as configs.
6. removed the implementation by @guillaumegenthial in favor of the cleaner implementation.
7. Interface changes: removed recursive and convert from the signature of instantiate. Removed support for instantiating a ListConfig (Top level config must be a DictConfig, plain dict or dataclass or instance compatible with OmegaConf.