Make default numbers fully tabular, add fuller pnum feature, and other tiny fixes

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The main change here is that the default characters for all instances (Sans included) are now fully tabular, for better use in data tables, numerical animations, invoices, and more. They were always intended to be tabular by default, but they were unintentionally given kerning that made them not-quite-tabular. The kerning that was present was useful, so it was moved into a fuller proportional numbers (pnum) feature, and can still be activated for better text flow.

Other small changes and notes:

• This adds a minus.superior glyph, for negative exponents
• This fixes the width of the prebuilt ½ glyph in Sans styles (Closes v1.079 - 00BD onehalf - Advance width is only 600 #479)
• This adds a ratio glyph (Closes character request: ∶ #480)
• This adds mu (μ) as a greek character. It was already there under the mu.math Unicode value, but this will make it easier to find and use (Closes character request: Greek mu (μ) #481)
• This fixes not-quite-tabular figures (Closes Not-quite-tabular figures: kerning between period and default 7 #482)