speed up file compiles by using cached namespace #82

Take caching-namespace technique used in pollen/render, abstract it to new module, and adapt the technique for use in pollen/cache when loading doc and metas from a source file.

Take caching-namespace technique used in pollen/render, abstract it to new module, and adapt the technique for use in pollen/cache when loading `doc` and `metas` from a source file.