Add BitArray HasAllSet, HasAnySet and PopCount

[SimonCropp]

Adds the three BitArray bit-population members:

• HasAllSet() and HasAnySet(), added in net8
• PopCount(), added in net11

BitArray had no polyfills at all, so this adds the section. API count 1052 to 1055.

Semantics

Checked against net11 rather than assumed:

	empty	all clear	all set	mixed
HasAllSet()	true	false	true	false
HasAnySet()	false	false	true	true
PopCount()	0	0	length	count

The one that could catch an implementation out is HasAllSet() on an empty BitArray, which is true because the condition is vacuous.

The tests parameterise over lengths of 31, 32, 33, 64 and 100, since BitArray stores bits in an int[] and the unused high bits of the last word must not be counted. Each case is also checked with the final bit cleared.

The BCL implementations work a word at a time and these are bit at a time, but both are linear in the length, so no //Note: was added.

Verification

• Solution builds clean in Release. Consume builds clean in Debug across all 22 target frameworks.
• Tests green on net11.0 (1645), net462 (1622), net8.0 (1642) and net10.0 (1645), plus PublicTests, EmbeddedTests, UnsafeTests, NoRefsTests and NoExtrasTests.
• net8.0 is a useful case here, since it exercises the BCL for HasAllSet and HasAnySet and the polyfill for PopCount, against the same assertions.