Return to using cents for amounts and change to dollars on output, to avoid any possibility of floating point math weirdness

Problem/Motivation

The ff58e6e77aaee83058aa372dc41ba3cc757451a3 megacommit changed storage from cents to float.

As PHP documentation notes:

Warning Floating point precision

Floating point numbers have limited precision. Although it depends on the system, PHP typically uses the IEEE 754 double precision format, which will give a maximum relative error due to rounding in the order of 1.11e-16. Non elementary arithmetic operations may give larger errors, and, of course, error propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like `0.1` or `0.7`, do not have an exact representation as floating point numbers in base 2, which is used internally, no matter the size of the mantissa. Hence, they cannot be converted into their internal binary counterparts without a small loss of precision. This can lead to confusing results: for example, `floor((0.1+0.7)*10)` will usually return `7` instead of the expected `8`, since the internal representation will be something like `7.9999999999999991118...`.

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality.

Proposed resolution

Since we only need two decimal places of precision, restore our previous approach of storing monetary amounts in cents.

(Commerce module introduces its own Price and Calculator classes because it has to worry about sales tax and other things that breaks "whole cents" but that is not currently a concern we have.)

Remaining tasks

User interface changes

API changes

Data model changes