feat(specs): make environment specs managed attributes

[aar65537]

closes #98

• Implements private _*_spec properties on the Environment class. The properties are accessed through *_spec managed attributes implemented with the @property decorator. Properties are initialized with _make_*_spec() methods during Environment initialization.
• Implements generic ActionSpec type on the Environment class in order to statically type the action_spec attribute.
• Updates all environments to conform with changes to Environment class.
• Implements unit tests to ensure Environment specs can be accessed from a jitted function.
• Changes all references to *_spec() method to *_spec attribute.

Edit: now also closes #221

• Implements generic Observation type on Environment
• Implements generic types on Wrapper classes, so that Wrappers will inherit type hints from the Environments they wrap

[sash-a]

Sorry for the radio silence, I still want to get this in! What do you think about using cached_properties instead of properties as the spec should never change?

[aar65537]

I think cached_properties should work. The only issue I foresee is you might have to access the properties in the __init__ method to ensure the arrays are initialized outside of a jitted function.

[sash-a]

I think cached_properties should work. The only issue I foresee is you might have to access the properties in the __init__ method to ensure the arrays are initialized outside of a jitted function.

True, but we know what the cached properties are, so we could call them in the base env classes init?

[sash-a]

Also just a heads up, we're making a push to get sliding tile puzzle and flatpack envs merged before the ICLR camera ready deadline (about a week). Once this is done we'll get to this PR :smile:

[aar65537]

Thanks for the heads up. I changed all of the environments to use cached_property. I think it is more succinct than my original implementation. Thank you for the suggestion.

[aar65537]

I've implemented the changes for the SlidingTilePuzzle and cleaned up the last references to action_spec() in the docs. That should be all the environments.