Support parsing of multiple statements (5/5)

Parser::parse_sql() can now parse a semicolon-separated list of
statements, returning them in a Vec<SQLStatement>.

To support this we:

  - Move handling of inter-statement tokens from the end of individual
    statement parsers (`parse_select` and `parse_delete`; this was not
    implemented for other top-level statements) to the common
    statement-list parsing code (`parse_sql`);

  - Change the "Unexpected token at end of ..." error, which didn't have
    tests and prevented us from parsing successive statements  ->
    "Expected end of statement" (i.e. a delimiter - currently only ";" -
    or the EOF);

  - Add PartialEq on ParserError to be able to assert_eq!() that parsing
    statements that do not terminate properly returns an expected error.

tests/sqlparser_postgres.rs

@@ -372,13 +372,20 @@ fn verified_stmt(query: &str) -> SQLStatement {
 /// converting AST back to string equals to `canonical` (unless an empty string
 /// is provided).
 fn one_statement_parses_to(sql: &str, canonical: &str) -> SQLStatement {
-    let only_statement = Parser::parse_sql(&PostgreSqlDialect {}, sql.to_string()).unwrap();
+    let mut statements = parse_sql_statements(&sql).unwrap();
+    assert_eq!(statements.len(), 1);
+
+    let only_statement = statements.pop().unwrap();
     if !canonical.is_empty() {
         assert_eq!(canonical, only_statement.to_string())
     }
     only_statement
 }
 
+fn parse_sql_statements(sql: &str) -> Result<Vec<SQLStatement>, ParserError> {
+    Parser::parse_sql(&PostgreSqlDialect {}, sql.to_string())
+}
+
 fn parse_sql_expr(sql: &str) -> ASTNode {
     debug!("sql: {}", sql);
     let mut parser = parser(sql);