feat: reduce CrcReader::sum calls in GzEncoder::read_footer

[CosminPerRam]

Reduces calls to CrcReader::sum (which clones 16 bytes).
Replaces manual shifting with rust-provided to_le_bytes.
Moved explicit reference from declaration to usage.

I think this would technically be a perf improvement, but there is no real world difference in it at all (godbolt here).

[Byron]

Thanks a lot!

It's interesting that the optimizer seems to do all that anywhere so this change doesn't lead to any differences after all, despite of course, being easier to read, a big win in my book.