std.DynamicBitSetUnmanaged: respect padding bits

[travisstaloch]

EDITED

closes #19933

This patch makes setAll() preserve padding bits.

DynamicBitSetUnmanaged depends on its padding bits being zeroed as
stated in the 'masks' field's doc comments. but setAll() was setting
all masks, including the padding, to ones.

[nurpax]

The PR 22289 mentioned above adds unsetAll() which IMO is pretty commonly needed. Of course it can be accessed through bitset.unmanaged but that's inconsistent with a bunch of other methods that have a wrapper that calls into unmanaged like isSet(), count(), etc. That PR also fixes the same bug that's fixed here. I don't mind which of these goes in but clearly both of these PRs are improving the DynamicBitSet and not making it worse.