Ensure memory-efficient high-level document symbol cache #944

* Introduce SymbolBody representation (compact), from which the
    body can be reconstructed, rendering body computation lazy
  * Introduce SymbolBodyFactory for the creation of such instances,
    ensuring that instances created from the same factory share
    the same file content, allowing the in-memory and on-disk caches
    to have a significantly smaller memory footprint
  * Change type of "body" key in UnifiedSymbolRepresentation to use
    SymbolBody instead of str
  * Adjust all usages of the "body" key

test/solidlsp/elixir/test_elixir_symbol_retrieval.py

@@ -53,7 +53,7 @@ class TestElixirLanguageServerSymbols:
             assert containing_symbol["name"] == "def create_user(pid, id, name, email, roles \\\\ [])"
             assert containing_symbol["kind"] == SymbolKind.Method or containing_symbol["kind"] == SymbolKind.Function
             if "body" in containing_symbol:
-                assert "def create_user" in containing_symbol["body"]
+                assert "def create_user" in containing_symbol["body"].get_text()
 
     @pytest.mark.parametrize("language_server", [Language.ELIXIR], indirect=True)
     def test_request_containing_symbol_module(self, language_server: SolidLanguageServer) -> None: