[SECURITY] properly copy rdrand bytes into the buffer.

7 of 8 bytes were not copied to the buffer, as the buffer
was written to as uint8_t instead of uint64_t,
leading to many NUL bytes in the entropy pool.

I'll never cleanup code after testing.
I'll never cleanup code after testing.
I'll never cleanup code after testing.
I'll never cleanup code after testing.
spurious space
add and export a grabEntropyIO for different cases.

grabEntropy might just be too unsafe, it's probably a good idea to export
the grabEntropyIO primitive anyway.
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