WIP: Refactor hooks

Closed Sylvain Henry requested to merge hsyl20/ghc:hsyl20/refactor/hooks into master

Use Dynamic to avoid unwanted module dependencies with hooks. Each Hook now has its own datatype (e.g. RunMetaHook).

Parser dependency count (cf CountParserDeps test) decreases from 205 to 176.

Edited by Sylvain Henry

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