Remove all `@inbounds` – this is not a hotspot, nor sufficiently safe

When you're doing this much I/O, bounds checking is a completely trivial
overhead, so having `@inbounds` annotations everywhere is an unnecessary
optimization. There are also lots of instances in this code using indexes
that are by no means certain to be in bounds. I just encountered one that
was causing very hard to debug problems as it was basically trashing the
surrounding memory.
Consistently use = with spaces in for when the RHS is a range.
Assign NA mask unconditionally as `ind < 0`.
Use normal array assignment instead of arrayset.

The arrayset intrinsic is not intended for public usage. The use
of unsafe_copy! is also, well, unsafe.
Merge pull request #46 from StefanKarpinski/pull-request/b26f04d8

remove various unsafe operations