Add TextRange to Identifier (#8)

## Summary

This PR adds `TextRange` to `Identifier`. Right now, the AST only
includes ranges for identifiers in certain cases (`Expr::Name`,
`Keyword`, etc.), namely when the identifier comprises an entire AST
node. In Ruff, we do additional ad-hoc lexing to extract identifiers
from source code.

One frequent example: given a function `def f(): ...`, we lex to find
the range of `f`, for use in diagnostics.

Another: `except ValueError as e`, for which the AST doesn't include a
range for `e`.

Note that, as an optimization, we avoid storing the `TextRange` for
`Expr::Name`, since it's already included.

parser/src/function.rs

@@ -90,7 +90,7 @@ pub(crate) fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentLis
             Some((start, end, name)) => {
                 // Check for duplicate keyword arguments in the call.
                 if let Some(keyword_name) = &name {
-                    if !keyword_names.insert(keyword_name.clone()) {
+                    if !keyword_names.insert(keyword_name.to_string()) {
                         return Err(LexicalError {
                             error: LexicalErrorType::DuplicateKeywordArgumentError(
                                 keyword_name.to_string(),
@@ -103,7 +103,7 @@ pub(crate) fn parse_args(func_args: Vec<FunctionArgument>) -> Result<ArgumentLis
                 }
 
                 keywords.push(ast::Keyword {
-                    arg: name.map(ast::Identifier::new),
+                    arg: name.map(|name| ast::Identifier::new(name, TextRange::new(start, end))),
                     value,
                     range: TextRange::new(start, end),
                 });