Cursor doesn't read accross bytes

[6293]

https://github.com/AluVM/rust-aluvm/blob/eb4fcc41c5f01bbba1d9b6494227f0d9096429c0/src/isa/bytecode.rs#L950-L955

Here, reg spans across 2 bytes (last bit of first byte to third bit of second byte). write_u4, however, does not handle such cases https://github.com/AluVM/rust-aluvm/blob/866f9c8abd409578b8c34e047f581919dd800517/src/library/cursor.rs#L129

[dr-orlovsky]

Very good point. I think this may be also the case for other serialization

[6293]

Here is our current rule:

• multiple bytes are stored in little-byte, big-bit endian
  • e.g. 258 is stored as [00000010, 00000001]
• When multiple data are stored in a single byte, firstly stored data is in the lowest bits
  • e.g. if reg is 3/8 and index is 7/32, byte code looks like 00111_011
• We have no rule when a single data are stored across multiple bytes