rework struct initialization from read_json()

[mlin]

Initializing a struct from unsafe read_json(), change the transient intermediate codepath to go through Object instead of Map, avoiding various problems caused by spurious attempt to unify heterogeneous value types. Improve error messages.

mostly addresses #524

[coveralls]

Pull Request Test Coverage Report for Build 1479461576

• 72 of 74 (97.3%) changed or added relevant lines in 3 files are covered.
• 109 unchanged lines in 10 files lost coverage.
• Overall coverage decreased (-1.4%) to 95.064%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Type.py	2	3	66.67%
WDL/Value.py	60	61	98.36%

Files with Coverage Reduction	New Missed Lines	%
WDL/runtime/task.py	1	95.24%
WDL/runtime/workflow.py	1	97.21%
WDL/init.py	3	93.2%
WDL/runtime/error.py	4	93.33%
WDL/Value.py	4	98.02%
WDL/runtime/task_container.py	7	93.37%
WDL/Error.py	8	89.94%
WDL/Type.py	8	94.91%
WDL/Expr.py	23	94.02%
WDL/Tree.py	50	93.53%

Totals
Change from base Build 1432387464:	-1.4%
Covered Lines:	6818
Relevant Lines:	7172

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💛 - Coveralls

[mlin]

Coveralls seems to be counting ALL docstrings as newly uncovered 👎

[mlin]

Initializing a struct from unsafe read_json(), change the transient intermediate representation to Object instead of Map, avoiding spurious attempts to unify heterogeneous value types. Improve error messages.

addresses most of #524 @jlumpe