Translate fat arrow defns into "where" clauses.

[tcr]

e.g.

promote
    :: (Pretty node, Pos node)
    => node
    -> (Rust.Expr -> Rust.Expr -> Rust.Expr)
    -> Result -> Result -> EnvMonad s Result

[tcr]

I didn't fix this fully in my last commit—where clauses are still not generated yet.