Cache parsed YAML and JSON with environment life-cycle.

Parsing YAML and JSON files is a slow process and classic Hiera used a FileCache class that would store the parsed result indefinitely but discard it in case the inode, mtime, or size of the source file changed.

Hiera 5 must have a similar cache and it should share the environment life-cycle rather than being indefinite. The cache should be made available to a data provider function in a similar way that the classic Hiera does. By using a method on the Context that takes the path as an argument and expects a block to produce the parsed result for caching. The block is then only called when the file has changed.