Uint shift operations: incorrect overflow flags and truncated shift amounts

`Uint::overflowing_shl`/`overflowing_shr` returned false-negative overflow
flags. `overflowing_shl` missed bits shifted above `BITS` but within the top
limb (non-limb-aligned widths such as `U160`), and limbs wholly discarded by
shifts >= 64; `overflowing_shr` missed wholly discarded low limbs. Shifted
values were correct; only the flag was wrong.

The wrong flag propagates: `checked_shl`/`checked_shr` return `Some` instead
of `None`, `strict_*` fail to panic, and `saturating_*` return a wrapped
value instead of saturating. The incorrect `checked_shl` result causes
`to_base_be` (and string formatting) to loop forever on no-alloc builds for
non-limb-aligned widths — a denial of service if formatting is reachable
from untrusted input.

Separately, `wrapping_shl`/`wrapping_shr` on 64/128/256-bit types truncated
the shift amount modulo 2^32, so shifts >= 2^32 returned an incorrectly
wrapped value instead of zero; on 32-bit targets the generic path also
truncated 64-bit shift amounts.

Callers using checked or saturating shift semantics on untrusted shift
amounts may compute incorrect results.