Make type errors about effectfulness more obvious

[smores56]

Today, the following code produces a difficult to understand error:

module [broken, func!]

func! : Str => Result Str []

broken = List.map [] func!

When run against roc check:

── TYPE MISMATCH in test.roc ───────────────────────────────────────────────────

This 2nd argument to map has an unexpected type:

5│  broken = List.map [] func!
                         ^^^^^^^^^^

This func! value is a:

    Str => Result Str []

But map needs its 2nd argument to be:

    Str -> Result Str []

────────────────────────────────────────────────────────────────────────────────

1 error and 0 warnings found in 30 ms.

This tells us that the first function is effectful with a => and the second is pure with a ->. This is hard to distinguish at a glance, we'd like to make it more obvious. It would be nice for the error to say something along the lines of "This function is effectful, but it needs to be pure".

[Anton-4]

Having ^^ below the differing chars (=> and ->) would be nice as well.