read_json() stub and Any type

[mlin]

read_json() can return values of different types depending on what's in the JSON file at runtime. Introduce WDL.Type.Any to model this, and use it instead of None to represent the item types of empty Array/Map literals.

[coveralls]

Pull Request Test Coverage Report for Build 313

• 23 of 32 (71.88%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 95.114%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Type.py	17	19	89.47%
WDL/StdLib.py	2	9	22.22%

Files with Coverage Reduction	New Missed Lines	%
WDL/Type.py	2	95.93%

Totals
Change from base Build 307:	0.04%
Covered Lines:	2122
Relevant Lines:	2231

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💛 - Coveralls

[mckinsel]

IIRC, we ran into some funky wdl4s limitations with read_json for the HCA. See here for the discussion. Not sure how the deprecation of Object affects this.

[mlin]

@mckinsel thanks for the heads up -- right now we're just at the stage of stubbing out function signatures, so we'll need to come back to this, now that the spec is +Struct and -Object!