Move syntax things from Dhall.Core to a new module Dhall.Syntax

[sjakobi]

This partially addresses #1127.

[Gabriella439]

@sjakobi: I think my main question is: what is the criterion for what belongs in Dhall.Syntax vs. Dhall.Core? For example, some of the utilities in Dhall.Syntax seem like they could reside in Dhall.Core (like denote or subExpressions)

[sjakobi]

For denote, the motivation was to have only a single import cycle with Dhall.Pretty.Internal. Dhall.Syntax already has a hs-boot file. If denote would have stayed in Dhall.Core, we'd need to have an additional hs-boot file for Dhall.Core.

For the optics, the reasoning was that these always change together with the types, so I chose to locate them as low as possible in the module hierarchy.

I can document this in the module header.

Note that I plan to move the normalization code to another new module Dhall.Normalize. Dhall.Core would then only contain a few "miscellaneous" things, and re-export Dhall.Syntax and Dhall.Normalize.