Refactor GHCi UI to fix #11606, #12091, #15721, #16096
Instead of parsing and executing a statement or declaration directly we now parse them first and then execute in a separate step. This gives us the flexibility to inspect the parsed declaration before execution. Using this we now inspect parsed declarations, and if it's a single declaration of form `x = y` we execute it as `let x = y` instead, fixing a ton of problems caused by poor declaration support in GHCi. To avoid any users of the modules I left `execStmt` and `runDecls` unchanged and added `execStmt'` and `runDecls'` which work on parsed statements/declarations.
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