Reorganize emscripten benchmarks.

This is a first pass and only includes the Emscripten output. I didn't want to make more invasive changes (like moving source materials around) without some conversation as to how it should be handled.

(In short I was thinking a top level directory for each library, complete with make file. In some cases that may result in duplicate code, but I think it becomes much more maintainable overall that way.)