Lexer: Suggest adding 'let' on unexpected '=' token

Summary:
I've heard numerous fledgling Haskeller's complain about the behavior of
ghci regarding bindings. While most REPLs accept bindings of the form
`x = 42`, GHCi is implicitly a `do` block, meaning that the user must
know to use a `let` to introduce a binding.

Here we suggest to the user that they may need a `let` and give them a
small example in the event that we find an unexpected `=` token.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D980

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