C23 stdbit.h quick reference

The stdc_first_leading_ functions are slightly slippery, and require a careful reading of the standard. For example, stdc_first_leading_one is defined as:

The most significant index is the 0-based index counting from the most significant bit, 0, to the least significant bit, w − 1, where w is the width of the type that is having its most significant index computed.

The initial patches for these functions in musl got this wrong, instead using the more intuitive definition of most significant index.

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