StdLib overhaul

[mlin]

Refactor StdLib (#14) in anticipation of needing to provide context-dependent typechecking/implementation logic for various functions. For example, numerous functions are tightly intertwined with the task execution environment and in some cases don't even make sense elsewhere (e.g. stdout()).

[coveralls]

Pull Request Test Coverage Report for Build 570

• 148 of 155 (95.48%) changed or added relevant lines in 4 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.2%) to 96.496%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/Expr.py	47	48	97.92%
WDL/StdLib.py	82	88	93.18%

Files with Coverage Reduction	New Missed Lines	%
WDL/Expr.py	1	98.04%
WDL/_parser.py	3	94.84%

Totals
Change from base Build 565:	0.2%
Covered Lines:	2919
Relevant Lines:	3025

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💛 - Coveralls

[mlin]

oddly unrelated coverage loss: https://coveralls.io/builds/22983289/source?filename=WDL%2F_parser.py#L756