This is an alternative to #13242 which keeps the `ReadLE64` part, but moves the `crypto/common.h` dependency into `crypto/common.h` as a function outside of `uint256`.

**Reason:** this change will remove dependencies for `uint256` to `crypto/common.h`, `compat/endian.h`, and `compat/byteswap.h`.

This PR removes the need to update tests to be endian-aware/-independent, but keeps the (arguably dubious) `ReadLE64` part (which was only introduced to fix the tests, not for any functionality).

make check