Use const fn

[lrazovic]

This little PR aims to:

• Make u128_mulhi a const function.
• Move a common line of code before the if.
• Use the exclude field in cargo.toml to decrease the crate size.

[Kogia-sima]

Is there any reason for making a private method const function?

[Kogia-sima]

Closing as stale. Please reopen if you'd like to work on this further.