WDL 2.0 struct literals

[mlin]

Implements #113 openwdl/wdl#297

New struct literal syntax similar to the old object syntax, but improves static typechecking by including the intended struct type name in the literal. So we can typecheck the members before later coercing them to a struct declaration/input. (The object syntax still works.)

Some detailed surgery to make declared struct types available to Expr typechecking logic, where before they'd only been needed in Tree.

[coveralls]

Pull Request Test Coverage Report for Build 2229

• 61 of 61 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 96.764%

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Totals
Change from base Build 2221:	0.01%
Covered Lines:	6339
Relevant Lines:	6551

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💛 - Coveralls