Delete travis config, move tests to github actions.

rotate_{left,right} -> rotate_lanes_{left,right}

* add `Simd::from_slice`

uses a zeroed initial array and loops so that it can be const.
unfortunately, parameterizing the assert with slice length
needs `#![feature(const_fn_fn_ptr_basics)]` to work.

This changes simd_swizzle! to a decl_macro to give it a path,
so it can be imported using a path and not the crate root.
It also adds various uses that were missed and adjusts paths.

Co-authored-by: NJubilee <46493976+workingjubilee@users.noreply.github.com>

Co-authored-by: NJubilee <46493976+workingjubilee@users.noreply.github.com>

Expand swizzle API and migrate existing functions. Add rotate_left, rotate_right.

Hide implementation details

Add simd_shuffle macro

Co-authored-by: NCaleb Zulawski <caleb.zulawski@gmail.com>

Headings with # Safety and # Examples are more "std style".
Use terms like "enable" and "disable", rather than "mask" jargon.

Co-authored-by: NJacob Lifshay <programmerjake@gmail.com>

This unsafe variant allows the thinnest API, in case LLVM cannot
perform loop-invariant code motion on a hot loop when the safe
form is used.

An unchecked variant could be added to other forms, but doesn't
seem likely to improve anything, since it would just add heavier
codegen.

In a still-future edition, `unsafe_op_in_unsafe_fn` may error.
Let's get ahead of that.

Aligns module with rust-lang/library/core, creating an... unusual
architecture that is easier to pull in as a module, as core itself can
have no dependencies (as we haven't built core yet).

* Fix cargo features for nightly

Change vectors to be generic over element type.

Co-authored-by: NJubilee <46493976+workingjubilee@users.noreply.github.com>