Automatically generate "origin displacement unit"

[chiphogg]

When two units have a nonzero origin offset, that offset is basically an implicit unit. If we include it in common unit calculations, we will be able to produce the most efficient result for "common unit" calculations.

For example, consider computing ((0°C - 0 K) + (0°F - 0°C)). This table shows that mp-units produces an underlying value of 2298350, because it happens to store the temperature offset in millikelvins as an implementation detail. Au produces 45967, which is the most efficient possible result.

This may be challenging because mp-units has rejected using a "zero" type, whereas the origin displacement between any two units is usually zero. But this could probably be worked around.