Improve performance by not copying `Token`s as much

Part of https://github.com/apache/datafusion-sqlparser-rs/issues/1557

While looking at the flamegraph posted to that ticket, one obvious source of improvement is to stop copying each token so much.

Functions like `peek_token()`, and `expect_token()` copy the `Token` (which often includes a string)

```rust
impl Parser {
// returns an OWNED TokenWithLocation
pub fn peek_token(&self) -> TokenWithLocation {
...
}
}
```

For example
https://github.com/apache/datafusion-sqlparser-rs/blob/2e90e105a74bf9f50f2bad6c22992759ddb06880/src/parser/mod.rs#L3314-L3334

In the above code, the `non_whitespace.cloned()` call actually copies the token (and string) even when many callsites only need to check what it is and could get by with a &

## Suggestions for improvements

The biggest suggestion is to stop copying each token unless it actually needed a clone. For example instead of this

```rust
impl Parser {
// returns an OWNED TokenWithLocation
pub fn peek_token(&self) -> TokenWithLocation {
...
}
}
```

Make it like this

```rust
impl Parser {
// returns an reference to TokenWithLocation
// (the & means no copying!)
pub fn peek_token_ref(&self) -> &TokenWithLocation {
...
}
}
```

I think we could do this without massive breaking changes by adding new functions like `peek_token_ref()` that returned a reference to the token rather than a clone

## Ideas

I played around with it a bit and here is what I came up with. I think a similar pattern could be applied to other places

```rust
impl Parser {
...
/// Return the first non-whitespace token that has not yet been processed
/// (or None if reached end-of-file) and mark it as processed. OK to call
/// repeatedly after reaching EOF.
pub fn next_token(&mut self) -> TokenWithLocation {
self.next_token_ref().clone()
}

/// Return the first non-whitespace token that has not yet been processed
/// (or None if reached end-of-file) and mark it as processed. OK to call
/// repeatedly after reaching EOF.
pub fn next_token_ref(&mut self) -> &TokenWithLocation {
loop {
self.index += 1;
// skip whitespace
if let Some(TokenWithLocation {
token: Token::Whitespace(_),
location: _,
}) = self.tokens.get(self.index - 1) {
continue
}
break;
}
if (self.index - 1) < self.tokens.len() {
&self.tokens[self.index - 1]
}
else {
eof_token()
}
}
...
}

static EOF_TOKEN: OnceLock = OnceLock::new();
fn eof_token() -> &'static TokenWithLocation {
EOF_TOKEN.get_or_init(|| {
TokenWithLocation::wrap(Token::EOF)
})
}
```

Beginne in src/parser/mod.rs, insbesondere bei peek_token(), expect_token() und dem im Issue identifizierten Aufruf non_whitespace.cloned(). Verfolge, welche Aufrufstellen Tokens nur inspizieren und welche eigene Werte benötigen, und bewahre dabei das bestehende Parser-Verhalten, während unnötige Token-Kopien eliminiert werden; die Arbeit ist abgeschlossen, wenn diese Pfade redundantes Klonen vermeiden, ohne die bestehenden APIs zu beeinträchtigen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.