Add checked/overflowing/wrapping numeric casts

[digama0]

Currently, there are functions

fn checked_add(self, other: u8) -> Option<u8>
fn saturating_add(self, other: u8) -> u8
fn wrapping_add(self, rhs: u8) -> u8
fn overflowing_add(self, rhs: u8) -> (u8, bool)

for addition on u8. We should have this functionality also for casts, that is, functions like

fn checked_cast(self: u32) -> Option<u8>
fn saturating_cast(self: u32) -> u8
fn wrapping_cast(self: u32) -> u8
fn overflowing_cast(self: u32) -> (u8, bool)

with the obvious implementations. Specific traits for this functionality are up for debate.

[petrochenkov]

This is covered by rust-lang/rfcs#1218
It would be great if you described your use cases for these functions in the RFC comments.

[digama0]

Great! I'll go over there.