Rework typechecking of optional/nonempty quantifiers

[mlin]

Removes a few awkward workarounds / special cases for static validation of optional & nonempty type compatibility. They are now checked more rigorously by default, so for example any use of T? where T is expected requires an explicit coercion such as with select_first(). But to accommodate older WDLs which depend on less strictness, supplying WDL.load(..., check_quant=False) relaxes these static validation checks.

The WDL specification is vague as to how strict this typechecking should be; this compromise should let us accommodate older WDLs without necessarily reproducing every detail of what Cromwell does and doesn't enforce for different language versions

[coveralls]

Pull Request Test Coverage Report for Build 238

• 75 of 77 (97.4%) changed or added relevant lines in 6 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 94.61%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/StdLib.py	14	16	87.5%

Files with Coverage Reduction	New Missed Lines	%
WDL/Expr.py	1	95.81%

Totals
Change from base Build 224:	0.03%
Covered Lines:	1966
Relevant Lines:	2078

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 232

• 65 of 67 (97.01%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 94.595%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
WDL/StdLib.py	14	16	87.5%

Totals
Change from base Build 224:	0.01%
Covered Lines:	1960
Relevant Lines:	2072

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💛 - Coveralls