internal: remove accidental code re-use

FragmentKind played two roles:

* entry point to the parser
* syntactic category of a macro call

These are different use-cases, and warrant different types. For example,
macro can't expand to visibility, but we have such fragment today.

This PR introduces `ExpandsTo` enum to separate this two use-cases.

I suspect we might further split `FragmentKind` into `$x:specifier` enum
specific to MBE, and a general parser entry point, but that's for
another PR!

crates/parser/src/lib.rs

@@ -98,12 +98,8 @@ pub enum FragmentKind {
     Block,
     Visibility,
     MetaItem,
-
-    // These kinds are used when parsing the result of expansion
-    // FIXME: use separate fragment kinds for macro inputs and outputs?
     Items,
     Statements,
-
     Attr,
 }