Show valid substitutions for typed holes

The idea is to implement a mechanism similar to PureScript, where they
suggest which identifiers in scope would fit the given hole. In
PureScript, they use subsumption (which is what we would like here as
well). For subsumption, we would have to check each type in scope
whether the hole is a subtype of the given type, but that would require
`tcSubType` and constraint satisfiability checking. Currently,
`TcSimplify` uses a lot of functions from `TcErrors`, so that would
require more of a rewrite, I will hold on with that for now, and submit
the more simpler type equality version.

As an example, consider

```
ps :: String -> IO ()
ps = putStrLn

ps2 :: a -> IO ()
ps2 _ = putStrLn "hello, world"

main :: IO ()
main = _ "hello, world"
```

The results would be something like

```
    • Found hole: _ :: [Char] -> IO ()
    • In the expression: _
      In a stmt of a 'do' block: _ "hello, world"
      In the expression:
        do _ "hello, world"
    • Relevant bindings include
        main :: IO () (bound at test.hs:13:1)
        ps :: String -> IO () (bound at test.hs:7:1)
        ps2 :: forall a. a  -> IO () (bound at test.hs:10:1)
      Valid substitutions include
        putStrLn :: String
                    -> IO () (imported from ‘Prelude’ at
test.hs:1:1-14
                              (and originally defined in
‘System.IO’))
        putStr :: String
                  -> IO () (imported from ‘Prelude’ at
test.hs:1:1-14
                            (and originally defined in ‘System.IO’))
```

We'd like here for ps2 to be suggested as well, but for that we require
subsumption.

Reviewers: austin, bgamari, dfeuer, mpickering

Reviewed By: dfeuer, mpickering

Subscribers: mpickering, Wizek, dfeuer, rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3361

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