Interpolate when highways exit and reenter map boundaries

Cropping was leaving deleting the nodes, but leaving their keys in
highway node lists, which was breaking anything that needed
`node[highway.node[x]]`.

Tests files were also isolated in modules as the added tests behaved
differently when the file was run in isolation, and other existing
test files couldn't be run in isolation.
Merge pull request #30 from garborg/cropping

RFC: Interpolate Highways