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/mbe/src/tests.rs

@@ -3,10 +3,11 @@ mod rule;
 
 use std::fmt::Write;
 
-use ::parser::FragmentKind;
 use syntax::{ast, AstNode, NodeOrToken, SyntaxNode, WalkEvent};
 use test_utils::assert_eq_text;
 
+use crate::FragmentKind;
+
 use super::*;
 
 pub(crate) struct MacroFixture {