Improve `foldl`'s stability on nested Iterators (#45789)
* Make `Fix1(f, Int)` inference-stable
* split `_xfadjoint` into `_xfadjoint_unwrap` and `_xfadjoint_wrap`
* Improve `(c::ComposedFunction)(x...)`'s inferability
* and fuse it in `Base._xfadjoint`.
* define a `Typeof` operator that will partly work around internal type-system bugs
Closes #45715
