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.
set: refactor set.Strings to be an alias of map[string]bool
set: refactor set.Tags to be an alias of map[names.Tag]bool
Merge pull request #85 from ericsnowcurrently/filestorage-SetFileInfo

Metadata.SetFile -> Metadata.SetFileInfo.
Merge pull request #86 from juju/101-set-refactor

set: make set types aliases for maps
Revert "SetFile -> SetFileInfo."

We'll re-merge it after the set-related code is updated.

This reverts commit e7b7868dea3aad14476bc95245529cd5f39b790b.
Merge pull request #87 from ericsnowcurrently/revert-SetFileInfo

Revert "SetFile -> SetFileInfo."
Revert "Revert "SetFile -> SetFileInfo.""

This reverts commit fe902901d130b8f780f4f7c1e0127e15968e9a57.

It effectively re-merges e7b7868dea3aad14476bc95245529cd5f39b790b.
Merge pull request #88 from ericsnowcurrently/revert-revert-SetFileInfo

Revert "Revert "SetFile -> SetFileInfo.""
Fixed http transport for windows compatibility