Add Euler spiral curve representation

[raphlinus]

This commit adds support for piecewise Euler spirals, gated under the euler feature. A primary goal is to support parallel curves, as described in raphlinus/raphlinus.github.io#46.

There are some rough edges, and there's not an end-to-end parallel curve yet, but it's progress. Here's what's currently implemented:

• The EulerSeg curve, along with some ParamCurve traits.
• Geometric Hermite interpolation of an Euler spiral segment
• Conversion of CubicBez to an iterator of Euler spiral segments
• Conversion of an Euler spiral segment to an iterator of CubicBez

Here's what's missing:

• The actual parallel curve algorithm at the heart of the blog post
• An optimized version of Geometric Hermite interpolation (see blog post)
• Error bounds on the above
• ParamCurveNearest and ParamCurveExtrema on EulerSeg
• Most of EulerPath including the Shape trait
• And end-to-end driver

It's slightly unclear that we'll even develop EulerPath at this point. It's possible to exercise the main parallel curve flow by composing iterators, and ending up with a BezPath. Possibly that should be taken out of this PR and added later if/when needed.

[cmyr]

@raphlinus is this ready? going to do a piet release soon, might as well get whatever outstanding kurbo work merged as makes sense.

[raphlinus]

I don't think we should block a piet release on it. I want to make some tweaks to the API (take an accuracy parameter pretty much everywhere), put the euler stuff into an euler namespace instead of top level, and make sure things are labeled as experimental. I'm also increasingly unsure about the "euler" feature - it feels like there should be an "advanced" or "full" feature that would encompass this, #174, and other future stuff along similar lines.

I can get this ready but it would be tomorrow.

[cmyr]

no stress, I'm not going to get the release done today, although we're not too far off.