Fix ListParameter Hashability

[dlstadther]

Description

With the merge of #2115, ListParameter normalization uses the _recursively_freeze() method to transform lists of dictionaries into tuples of _FrozenOrderedDict. However, ListParameter's parse() and serialize() methods were not updated to utilize the _DictParamEncoder, allowing JSON serialization.

This PR resolves the serialization of lists of dictionaries provided to ListParameter.

Motivation and Context

Fixes unhashability of ListParameter when provided a list of dictionaries caused by #2115.

Have you tested this? If so, how?

Added hashability test for list of dictionaries.

[dlstadther]

@Tarrasch @j-ostrich Could y'all review? Thanks!

[jonathan-ostrander]

I think the same changes should be made for TupleParameter since its members can also be objects. Other than that, LGTM.

[dlstadther]

TupleParameter and ListParameter only differ in their parse() methods. However, i'm not sure if it makes sense to subclass one of them as the other. Tuples and Lists are similar of use, but not in design or purpose.

Should I subclass TupleParameter as a class of ListParameter, only overriding the parse() method?

[jonathan-ostrander]

I don't see the benefit of subclassing besides saving a couple of lines of code, but I'm also not against it. LGTM, but I don't have write access so you'll have to wait for @Tarrasch to take a look.

[dlstadther]

@joemeszaros I see you added DictParameter. Do you think you could review this? It would be much appreciated! Then i'll merge.

[dlstadther]

I'll merge this tomorrow (2017-09-12) unless I hear otherwise.

[joemeszaros]

@dlstadther Sorry for the late response, but I was on holiday and got back just now.

[dlstadther]

@joemeszaros No worries. Feel free to still review (although merged). If you have problems or suggestions, i'm happy to submit another PR with those changes.