Restructure compilation pipeline to allow hooks

This commit exposes GHC's internal compiler pipeline through a `Hooks`
module in the GHC API. It currently allows you to hook:

 * Foreign import/exports declarations
 * The frontend up to type checking
 * The one shot compilation mode
 * Core compilation, and the module iface
 * Linking and the phases in DriverPhases.hs
 * Quasiquotation

Authored-by: Luite Stegeman <stegeman@gmail.com>
Authored-by: Edsko de Vries <edsko@well-typed.com>
Signed-off-by: Austin Seipp <austin@well-typed.com>