[#648] Add dot operator support for Lorentz storage fields

Description

Add support for OverloadedRecordDot to StoreClass; deprecate StoreClass.Extra.

There's a small kink: since Lorentz is intended to work with RebindableSyntax, OverloadedRecordDot doesn't use GHC's HasField, but instead whatever getField is in scope. Lorentz provides its own getField, and its type isn't right, which will lead to confusing error messages like this:

• Expected a type, but
  ‘"gField"’ has kind
  ‘ghc-prim-0.9.0:GHC.Types.Symbol’

The workaround is to hide Lorentz's getField and import GHC's from GHC.Records. This is suboptimal, to say the least, but I couldn't think of any alternatives, save for renaming Lorentz's getField.

Related issue(s)

Resolves #648 (closed)

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