Don’t generate empty polygons during clipping.

Fixes #1823; spurious closePath events were being generated for
degenerate polygons due to generation of empty polygons and rings in
rare cases.
Optimize interpolateString.
Only coalesce exact string matches.

If there are a lot of matching numbers, it’s faster to do direct string equality
comparisons than it is to coerce to a number and compare numerically.
Add d3.interpolateString benchmark.
Run each benchmark separately.
Merge remote-tracking branch 'origin/geo-empty-polygon' into 3.4.5
Merge branch '3.4.5'
The quantile scale should ignore null, too.
Code formatting: fix indent.
Fix #1839 - sign of parsed timezone offset.
Optimise d3.mean.

Originally we were using Welford’s algorithm, but this is primarily
useful when computing the variance in a numerically stable manner, since
Welford’s approach requires an incremental mean.

I’ve removed a test for the mean of more than one instance of
Number.MAX_VALUE as this is unlikely to occur in practice; most likely
this was the reason I used Welford’s algorithm in the first place.

There’s a paper [1] comparing various algorithms for computing the mean,
and Welford’s is actually slightly less accurate than the naïve
approach.  There are some more accurate approaches but I think it’s
overkill for d3.mean.

[1] Youngs, Edward A., and Elliot M. Cramer. "Some results relevant to
choice of sum and sum-of-product algorithms." Technometrics 13.3 (1971):
657-665.

Related: #1842.
Merge branch 'fix-quantile-null' into 3.4.6
Merge branch 'fix-parse-timezone' into 3.4.6
Merge branch 'mean' into 3.4.6
Merge branch '3.4.6'
Fix #1850 - multiple touchstart targets.
removing .js extension from the 'main' property
Don’t store temporary variables on tree nodes.

Rather than creating a temporary _tree hash on the tree nodes to store temporary
variables needed to compute the tree layout, the tree is wrapped. This
eliminates the risk of a namespace collision, and eliminates the need to
subsequently delete temporary variables. (They will be garbage collected.)
Non-recursive tree layout.
Non-recursive hierarchy layout.