Window functions and experimental syntax promotion (#325)

[parsonsmatt]

• Remove r -> a fundep from SqlSelect and update to accept to support the change

• fix record derivation to support SqlSelect fun dep changes

• Move window definition from previous branch to new branch based off of current master; Break Frame definition out into its own file; Start Work on connecting Window definition language to SqlExpr language

• Fix typo with preceding; Add initial tests to verify window functions are rendering correctly

• Add window function tests

• Add new context variable to SqlExpr; Start with everything in terms of ValueContext and begin easing constraints

• Add aggregate context; Update all aggregation functions to return SqlAgg; Modify ToAliasReference to correctly reset context to value (all references are values); Added test to verify that filterWhere works with over

• Revert all SqlAgg functions back to SqlExpr; Split SqlSelect typeclass in two to support type inference for subquery context erasure

• Remove MergeContext in favor of explicitly lifting values into the windowed context

• Relax the type of aggregate functions to leave ctx open.

• Add some comments; Change the ctx on some functions

• Run stylish on window modules

• Fix implementation of ToAliasReference in record derivation

• Preserve type variable ordering of

• Add type role to forbid coerce

• fix build

• rename between between_

• export framerange type

• generalize case_

• generalize val

• a few more patches

• Refactor common tests

• Add legacy test modules for mysql and sqlite; Fix warnings in test compilation

• Add changelog entry; Add warnings to Experiment/Legacy top level modules; Change default top level module to new syntax

• Run stylish on window function modules

• Fix compilation error in warning pragmas

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• Documented new APIs with Haddock markup.
• Added @since declarations to the Haddock.
• Ran stylish-haskell and otherwise adhered to the style guide.

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