UnusedDeclaration and UnusedCall linters

[mlin]

UnusedDeclaration: a (non-output) value is declared, but not subsequently used in the task/workflow.
UnusedCall: the outputs of a call within a workflow are neither outputs from the workflow, nor used by anything else in the workflow.

In the process of developing these, we realized that the task commands in many draft-2 WDLs were not being parsed as expected (specifically, ${ } delimited placeholders were ignored within <<< >>> delimited commands, which are indeed not recognized as placeholders in WDL 1.0). To address this, we introduce a rudimentary way to serve different grammar versions, and detect the version from the document header, if any. The mechanism will need more polishing.

Typecheck task runtime section.

Fix several bugs and omissions in the AST, Walker, etc.

[coveralls]

Pull Request Test Coverage Report for Build 208

• 103 of 104 (99.04%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.4%) to 94.447%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Tree.py	13	14	92.86%

Totals
Change from base Build 199:	0.4%
Covered Lines:	1922
Relevant Lines:	2035

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 216

• 101 of 102 (99.02%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 94.329%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Lint.py	25	26	96.15%

Files with Coverage Reduction	New Missed Lines	%
WDL/Error.py	2	87.01%

Totals
Change from base Build 199:	0.2%
Covered Lines:	1913
Relevant Lines:	2028

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💛 - Coveralls