This type selects exactly one bit in a register. It is a BitPos shifted from
a counter to a selector, and is used to apply test and write operations to real
memory.
R: The register element this selector governs.
Values of this type are required to have exactly one bit set and all others cleared. Any other value makes the program incorrect, and will cause memory corruption.
This type is only constructed from BitPos, and is always equivalent to
1 << BitPos.
The chain of custody from known-good BitIdx values, through proven-good
BitOrder implementations, into BitPos and then BitSel proves that values
of this type are always correct to apply to real memory.