For std::copy to optimize a copy to memcpy, the valuetype must be both
trivially constructable and trivially copyable.

The new copy benchmarks highlighted an issue that std::copy'ing pairs
and vecs were not optimized to memcpy. For a 256 MiB buffer on my
laptop w/ GCC, the serial copy speeds were:

UInt8:                 10.10 GiB/s
Vec<UInt8, 2>           3.12 GiB/s
Pair<UInt32, Float32>   6.92 GiB/s

After this patch, the optimization occurs and a bitwise copy occurs:

UInt8:                 10.12 GiB/s
Vec<UInt8, 2>           9.66 GiB/s
Pair<UInt32, Float32>   9.88 GiB/s

Check were also added to the Vec and Pair unit tests to ensure that
this classes continue to be trivial.

The ArrayHandleSwizzle test was refactored a bit to eliminate a new
'possibly uninitialized memory' warning introduced with the default
Vec ctors.