For enums that describe a bitmask where successive bits within that mask
describe some enum value (as described in the same enum), it is useful
to support operator<< and operator>> as well.
For example:
```
enum class E : unsigned {
// 2 bits per option
OptionA = 0,
OptionB = 1,
OptionC = 2,
OptionD = 3,
OptionMask = 3,
// Given 3 values in the bitmask X, Y and Z, each is 2 bits in size
// and represents a choice of OptionA..OptionD.
ShiftX = 0,
ShiftY = 2,
ShiftZ = 4,
};
// The mask can be encoded with:
E mask;
mask |= getOptionFor(X) << E::ShiftX;
mask |= getOptionFor(Y) << E::ShiftY;
mask |= getOptionFor(Z) << E::ShiftZ;
// And to extract a value:
E OptionForX = (mask >> E::ShiftX) & E::OptionMask;
E OptionForY = (mask >> E::ShiftY) & E::OptionMask;
E OptionForZ = (mask >> E::ShiftZ) & E::OptionMask;
```
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