Introduce SQLObjectName struct (4.1/4.4)

(To store "A name of a table, view, custom type, etc., possibly
multi-part, i.e. db.schema.obj".)

Before this change

  - some places used `String` for this (these are updated in this commit)

  - while others (notably SQLStatement::SQLDelete::relation, which is
    the reason for this series of commits) relied on
    ASTNode::SQLCompoundIdentifier (which is also backed by a 
    Vec<SQLIdent>, but, as a variant of ASTNode enum, is not convenient
    to use when you know you need that specific variant).

src/sqlparser.rs

@@ -1060,11 +1060,11 @@ impl Parser {
         }
     }
 
-    pub fn parse_tablename(&mut self) -> Result<String, ParserError> {
+    pub fn parse_tablename(&mut self) -> Result<SQLObjectName, ParserError> {
         let identifier = self.parse_compound_identifier(&Token::Period)?;
         match identifier {
             // TODO: should store the compound identifier itself
-            ASTNode::SQLCompoundIdentifier(idents) => Ok(idents.join(".")),
+            ASTNode::SQLCompoundIdentifier(idents) => Ok(SQLObjectName(idents)),
             other => parser_err!(format!("Expecting compound identifier, found: {:?}", other)),
         }
     }