dev: various optimizations

[enitrat]

Optimizes ToBytes, FromBytes, to_u64_words

[PASS] utils::traits::integer::tests::u32_test::test_u32_to_bytes_full (gas: ~5)
        steps: 1382
        memory holes: 53
        builtins: (range_check: 125, bitwise: 19)
        syscalls: ()

to

[PASS] utils::traits::integer::tests::u32_test::test_u32_to_bytes_full (gas: ~2)
        steps: 442
        memory holes: 20
        builtins: (range_check: 29, bitwise: 4)
        syscalls: ()

[PASS] utils::traits::integer::tests::u32_test::test_u32_to_le_bytes_full (gas: ~5)
        steps: 1349
        memory holes: 53
        builtins: (range_check: 117, bitwise: 19)
        syscalls: ()

to

[PASS] utils::traits::integer::tests::u32_test::test_u32_to_le_bytes_full (gas: ~2)
        steps: 486
        memory holes: 20
        builtins: (range_check: 34, bitwise: 4)
        syscalls: ()

[PASS] utils::traits::bytes::tests::span_u8_test::test_span_u8_to_64_words_full (gas: ~11)
        steps: 2739
        memory holes: 141
        builtins: (range_check: 274, bitwise: 39)
        syscalls: ()

[PASS] utils::traits::bytes::tests::span_u8_test::test_span_u8_to_64_words_partial (gas: ~8)
        steps: 1965
        memory holes: 114
        builtins: (range_check: 193, bitwise: 27)
        syscalls: ()

to

[PASS] utils::traits::bytes::tests::span_u8_test::test_span_u8_to_64_words_partial (gas: ~8)
        steps: 1972
        memory holes: 81
        builtins: (range_check: 199, bitwise: 27)
        syscalls: ()

[PASS] utils::traits::bytes::tests::span_u8_test::test_span_u8_to_64_words_full (gas: ~12)
        steps: 2847
        memory holes: 146
        builtins: (range_check: 285, bitwise: 39)
        syscalls: ()

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This change is 

[obatirou]

you could use rem if not mistaken
https://github.com/starkware-libs/cairo/blob/a74ebfdf7da7bf31c124b7d2fa0cdb39fd1bcb68/corelib/src/integer.cairo#L2138

[obatirou]

Why not keep default ?
More explicit to have an empty array macro ?