This commit extends the `BitSet` class with several new
methods:
- `CountElements()`
- `IsSubsetOf()` and `IsSupersetOf()`
- `Complement()`
- `UnionWith()`, `IntersectWith()`, and `SubtractWith()`
- `SetMask()`, `AppendTo()`, and `ReadFrom()` message.
This commit also introduces a new `BitSetUtils` non-template base class
for the `BitSet<kNumBits>` template class. This change helps optimize
code by moving the common implementation logic for various bit
manipulation operations out of the template, reducing template
instantiation overhead.
Abtin Keshavarzian