Make interface generic, Drop StaticArrays as hard dependency

[sjkelly]

This package really doesn't need StaticArrays for functionality. By removing it we greatly improve load times and also make the interface more generic, allowing arbitrary support for any StaticVector-like constructor.

before:

julia> @time using Contour
  0.842599 seconds (2.50 M allocations: 187.757 MiB, 0.37% compilation time)

after:

julia> @time using Contour
  0.020156 seconds (2.95 k allocations: 271.500 KiB, 31.36% compilation time)`

Coupled with and this and https://github.com/JuliaGeometry/GeometryBasics.jl/pull/169 , we make loading Plots almost 10% faster:

before: 
julia> @time using Plots
  5.908080 seconds (8.87 M allocations: 655.109 MiB, 7.27% gc time, 25.84% compilation time)

julia> @time using Plots
  5.541558 seconds (8.62 M allocations: 618.985 MiB, 6.66% gc time, 22.87% compilation time)

[sjkelly]

I am not 100% this is the API we want yet, but it is a step in the right direction for a point release.