Rename `as_str` to `to_str` (#8886)
This PR renames the method on `StringLiteralValue` from `as_str` to
`to_str`. The main motivation is to follow the naming convention as
described in the [Rust API
Guidelines](https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv).
This method can perform a string allocation in case the string is
implicitly concatenated.
