add exact bitshifts

[Qelxiros]

Tracking issue: #144336

cc @lolbinarycat

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[kennytm]

Commenting here to avoid cluttering the closed ACP or the tracking issue.

Regarding the names, we preferred calling these "exact" shifts just like exact_div

impl iX/uX {
    // Shift succeeds if `shift < Self::BITS` and right shifting the result returns the original value.
    fn exact_shl(self, shift: u32) -> Option<Self>;
    unsafe fn exact_shl_unchecked(self, shift: u32) -> Self;

If we're naming the methods after exact_div shouldn't there be 3 functions:

• checked_exact_shl for the version returning Option (ref 1)
• exact_shl for panicking (ref 2)
• unchecked_exact_shl for the unchecked unsafe version (ref 3)