This change removes support for `ghc-7.10`.  The feature we're using
was only added in `ghc-8.0.1`, and there are already a few other shims for
that older version of GHC.

This change uses explicit export lists in the generated proto modules (but
only for `Proto.Foo.Bar`, not `Proto.Foo.Bar'Fields`).  The same names should
be exported as before, the only difference is that for enum aliases:
```
enum Foo {
  option allow_alias = true;
  Foo = 1;
  Bar = 2;
  BarA = 2;
}
```
which produce the Haskell code:
```
data Foo = Foo | Bar
pattern BarA :: Foo
pattern BarA = Bar
```
we now export explicitly `Foo(.., BarA)`, rather than exporting `BarA` as a
separate name.  That way, if someone writes `import Proto.File (Foo(..))` then
they also get `BarA` automatically.

For example:

```
enum Foo {
  Foo0 = 0;
  Foo1 = 1;
}
```

becomes

```
newtype Foo = Foo Int32
pattern Foo0 :: Foo
pattern Foo0 :: Foo 0
pattern Foo1 :: Foo
pattern Foo1 :: Foo 1
```

Additionally, proto3 enums are exported from the `Proto.*` module with the
form `Foo(Foo0, Foo1)`, so that they can be imported as `Foo(..)` to get the
patterns.