Reimplement vec<> class

[illuhad]

Reimplement and simplify vec class to not rely on CUDA/HIP vector types which currently don't yield any performance benefits with clang anyway (note that, if there are performance benefits in the future we could still always specialize the vec_storage backend to use them for cases where it makes sense).

Key stats of this change:

• -3000 lines of code (!)
• +50% readability
• +100% maintainability
• +20% C++17 (old implementation was based on C++14 and hence did not use fold expressions)
• +/- 0% performance (expected)
• +10% spec conformance, e.g. alignments.
• +5% SYCL 2020 (implements SYCL 2020 additions to the vec interface)

[illuhad]

@hipSYCL-ci-bot test this please, my favorite bot!