Separate Additional Medicare Tax parameters by payroll / self-employment

#654 puts Additional Medicare Tax parameters (rate and exemptions) together, as they're the same, despite appearing (identically) in separate parts of the US Code.

The self-employment part of the code (https://www.law.cornell.edu/uscode/text/26/1401#b_2) indicates that the wages would be subtracted first, which aligns with [Form 8959](https://www.irs.gov/pub/irs-pdf/f8959.pdf) and how the code is structured.

Start by reviewing issue #654 and the existing Additional Medicare Tax parameter definitions, then compare the payroll and self-employment rules with 26 U.S.C. §1401(b)(2) and Form 8959. Done means payroll and self-employment parameters are separated and the self-employment wage subtraction follows the cited code and form.

Written by the indexing model from the issue text.