Template generics (#2200)

Add template generics, with optional constraints but no [SFINAE](https://en.cppreference.com/w/cpp/language/sfinae), to Carbon. Template generics allows the compiler postpone type checking of expressions dependent on a template parameter until the function is called and the value of that parameter is known.

Example usage:

```carbon
fn Identity[template T:! Type](x: T) -> T {
  return x;
}
```

Co-authored-by: Richard Smith <richard@metafoo.co.uk>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>