parallel: refactor Run

This makes it considerably faster and more memory efficient.
Merge pull request #83 from rogpeppe/005-faster-parallel-run

parallel: more efficient Run
remove unnecessary indirection on Remove
added warning
panic when set is not initialised
Merge pull request #84 from juju/100-fix-set-bugs

sets cannot be safely lazily initalised
SetFile -> SetFileInfo.
Merge pull request #85 from ericsnowcurrently/filestorage-SetFileInfo

Metadata.SetFile -> Metadata.SetFileInfo.