refactor drop glue and fix Box<Box<Struct>> as Box<Trait>

Skip C/Rust type checks for eventfd.

* Add single output _mm_cvt[t]ss_* variants

The *_pi variants are currently blocked by
rust-lang/stdarch#74

* Add _mm_cvtsi*_ss

The _mm_cvtpi*_ps intrinsics are blocked by
rust-lang/stdarch#74

* Fix Linux builds

Also the si64 variants are only available on x86_64

Blocks grammar

* Add from_bitmask
* Add mips workaround

Stop removing the llvm-asm tests in test.sh since they don't exist anymore

We now only have 12 uses of `Location::none` left, all of which have no
reasonable location to use.

Resolves: rust-lang#136

…ion (rust-lang#136)

Resolves rust-lang#72 

We added invariants for Nanoseconds and Duration to match the safety
conditions for those types.

We add safety requirements to the following methods:
- `new`, `from_secs`, `from_millis`, `from_micros`, `from_nanos`,
`as_secs`, `as_millis`, `as_micros`, `as_nanos`, `subsec_millis`,
`subsec_micros`, `subsec_nanos`, `checked_add`, `checked_sub`,
`checked_mul`, `checked_div`

We additionally add correctness conditions to the following methods:
- `from_secs`, `as_secs`, `subsec_millis`, `subsec_micros`,
`subsec_nanos`, `as_millis`, `as_micros`

Support for `kani::Invariant` depends on rust-lang#87. For the interim we
implemented a proxy trait `TempInvariant` that exposes the same
`is_safe` method. We will update this once rust-lang#87 is merged.

While the safety check for `Duration::as_nanos()` succeeds, we ran into
timeouts for `Duration::as_nanos()` when we tried to use a correctness
contract and we're looking for advice on how to speed up that
verification time. We were able to prove it for `u16::MAX`, but hit
timeouts for larger numbers.

We are unsure if the pre-conditions for `Duration::new()` are acceptable
because they limit the range of values that you can call
`Duration::new()` with. However, we think it's reasonable since we limit
the values to values that don't panic. Let us know if this is a thing
that we should change.

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 and MIT licenses.

---------

Co-authored-by: Cole Vick <[email protected]>
Co-authored-by: Celina G. Val <[email protected]>
Co-authored-by: Michael Tautschnig <[email protected]>