Support #![no_std]

[Shnatsel]

libdiffuzz doesn't make much use of the standard library. It can probably be switched to the corresponding libcore primitives and compiled in #![no_std] mode.

Among other things, this will reduce the size of the generated binary and may allow cross-compilation to the more obscure architectures.

[jhwgh1968]

To anyone visiting this issue in the future: my WIP PR did make it support no-std technically, but other limitations cropped up. Those need a solution before this can be closed.