Rename length to width/duration

[dlfivefifty]

length is used extensively in Base for iterator and broadcasting behaviour, and so overriding length(::ClosedInterval) to mean something other than the iterator length is a bad idea.

I'd propose calling it arclength.

[dlfivefifty]

Hmm, I forgot that I already did this in the PR as width.

[hyrodium]

The width was already added, and the method for length was removed in #92.