Merge #127

127: Improve folding r=matklad a=aochagavia

I was messing around with adding support for multiline comments in folding and ended up changing a bunch of other things. 

First of all, I am not convinced of folding groups of successive items. For instance, I don't see why it is worthwhile to be able to fold something like the following:

```rust
use foo;
use bar;
```

Furthermore, this causes problems if you want to fold a multiline import:

```rust
use foo::{
  quux
};
use bar;
```

The problem is that now there are two possible folds at the same position: we could fold the first use or we could fold the import group. IMO, the only place where folding groups makes sense is when folding comments. Therefore I have **removed folding import groups in favor of folding multiline imports**.

Regarding folding comments, I made it a bit more robust by requiring that comments can only be folded if they have the same flavor. So if you have a bunch of `//` comments followed by `//!` comments, you will get two separate fold groups instead of a single one.

Finally, I rewrote the API in such a way that it should be trivial to add new folds. You only need to:

* Create a new FoldKind
* Add it to the `fold_kind` function that converts from `SyntaxKind` to `FoldKind`

Fixes #113 

Co-authored-by: Adolfo Ochagavía <github@adolfo.ochagavia.xyz>

crates/ra_syntax/src/ast/generated.rs

@@ -2197,3 +2197,21 @@ impl<'a> WhileExpr<'a> {
     }
 }
 
+// Whitespace
+#[derive(Debug, Clone, Copy)]
+pub struct Whitespace<'a> {
+    syntax: SyntaxNodeRef<'a>,
+}
+
+impl<'a> AstNode<'a> for Whitespace<'a> {
+    fn cast(syntax: SyntaxNodeRef<'a>) -> Option<Self> {
+        match syntax.kind() {
+            WHITESPACE => Some(Whitespace { syntax }),
+            _ => None,
+        }
+    }
+    fn syntax(self) -> SyntaxNodeRef<'a> { self.syntax }
+}
+
+impl<'a> Whitespace<'a> {}
+