Explicit `as_str` (no deref), add no allocation methods (#8826)
## Summary
This PR is a follow-up to the AST refactor which does the following:
- Remove `Deref` implementation on `StringLiteralValue` and use explicit
`as_str` calls instead. The `Deref` implementation would implicitly
perform allocations in case of implicitly concatenated strings. This is
to make sure the allocation is explicit.
- Now, certain methods can be implemented to do zero allocations which
have been implemented in this PR. They are:
- `is_empty`
- `len`
- `chars`
- Custom `PartialEq` implementation to compare each character
## Test Plan
Run the linter test suite and make sure all tests pass.
