add deprecation warning

Author: PSeitz

src/lib.rs

@@ -186,6 +186,12 @@ pub use crate::postings::Postings;
 pub use crate::schema::DatePrecision;
 pub use crate::schema::{DateOptions, DateTimePrecision, Document, TantivyDocument, Term};
 
+#[deprecated("Will be removed in tantivy 0.23. Use export from snippet module instead")]
+pub use self::snippet::{Snippet, SnippetGenerator};
+
+#[deprecated("Will be removed in tantivy 0.23. Use export from indexer module instead")]
+pub use crate::indexer::{merge_filtered_segments, merge_indices, IndexWriter, PreparedCommit};
+
 /// Index format version.
 const INDEX_FORMAT_VERSION: u32 = 5;