Add "contextless" to procs in slice package

[thetarnav]

Looks like a lot of procedures in the slice package could use a "contextless" calling convention as most don't require context. Some procedures there already were contextless, but it was applied inconsistently—why would to_bytes be contextless, but not to_type.
Let me know if some of these should require context by design or something.

[Barinzaya]

The main consequence of this is that anything that can fail a bounds check (e.g. swap, split_first) will no longer be able to use context.assertion_failure_proc to allow for the panic behavior to be overridden (e.g. to "catch" the panic, or to log it).