I find the int.to_bytes() function that was used in var_int more readable than struct.pack and it is slightly faster too.

move-only from transaction.py

moved from test_bitcoin.py, the only change is var_int changed to write_compact_size

move-only from test_transaction.py

New tests

this block was doing a pointless bytes -> hex -> bytes conversion.

removed useless log line that avoid a hash computation and allows using serialize_sequence as a shortcut

I think it may be used in perf critical places. I wrote this function to estimate the transaction size or input size quickly without need to serialize it. The plan is to benefit from this in the CoinChooser to precompute a tx size and avoid selecting too many inputs (wrt to the tx relay size policy)

This method is actually writing the bytes to a buffer.
I should rename the write_compact_size to just compact_size, as the function is not writing anything anywhere, just returning bytes.

that's unrelated changes ?