Int-Uint conversion

[erik-3milabs]

This PR introduces Int::to_uint and Uint::to_int, allowing better conversion between these types.

[tarcieri]

Hmm, I'm used to as_* and to_* with similar names being symmetric operations which provide borrowed and owned conversions

[erik-3milabs]

Wdyt of

• sticking with as_(u)int for the casting operation (i.e., similar to u32 as i32), and
• using try_into_(u)int for the conversion operation, which may fail, hence the try_*

[erik-3milabs]

Hmmm, but try_into_(u)int is such a long name :(

And to_(u)int does match the to_odd and to_nz naming schemes 🤔