Inline constants for gregorian/3 form

This simplifies the code and opens the way for some mathematical
simplifications of some of these formulas.
Inline Julian Day variable

The Julian Day and Modified Julian Day have a simple relationship.  The
original source of these equations did the calculations in terms of
Julian Day, so that relation was just a placeholder for my convenience.
Inlining should make further simplifications possible.
Inline F0

Another step towards simplifying the Gregorian formulas.
Simplify F1 formula

I took the old formula, replaced MJD with x and pasted it into Wolfram
Alpha.  I then pasted one of its "Alternate Forms" here and replaced x
with MJD.  The simplified version has 4 arithmetic operations where the
old one had 10.  Incidental to the simpler equation, this should also
improve performance.
Inline F1 formula

The next simplification step.
Minor simplification of E

I tried a full simplification provided by Wolfram Alpha but several
tests failed.  I tried some variations which also failed.  This is the
simplest I could make the formula while still having clpfd calculate
results in agreement with the current test suite.
Inline G formula

The parentheses around the inlined term are necessary.  Without them,
the test suite fails.
Refactor to remove some parentheses

This just reduces the visual clutter a little.
Combine some constants

This is just a slight visual simplification.
Rearrange Month constraint

This might open the way for some extra simplification.