math.js consistency

[Fil]

I've checked a lot (maybe all) math.js files in src/ and found two inconsistencies in d3-shape:

• epsilon = 1e-12 (elsewhere it's 1e-6 and we have epsilon2 = 1e-12)
• tau = 2 * pi (elsewhere it's pi * 2)

https://beta.observablehq.com/@fil/math

[mbostock]

Not really sure if we need to do anything here.

[Fil]

I would suggest to rename epsilon as epsilon2 in this module.