Function definition in GHCi

This patch allows define and re-define functions in ghci. `let` is not
required anymore (but can be used).

Idea: If ghci input string can be parsed as statement then run it as
statement else run it as declaration.

Reviewers: mpickering, bgamari, austin

Reviewed By: mpickering, bgamari, austin

Subscribers: hvr, mpickering, dterei, thomie

Differential Revision: https://phabricator.haskell.org/D1299

GHC Trac Issues: #7253