Use ptr::metadata in <[T]>::len implementation
This avoids duplication of ptr::metadata code.
I believe this is acceptable as the previous approach essentially duplicated `ptr::metadata` because back then `rustc_allow_const_fn_unstable` annotation did not exist.
I would like somebody to ping `@rust-lang/wg-const-eval` as the documentation says:
> Always ping `@rust-lang/wg-const-eval` if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.
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