minor mailcap and entity command refactors

[frasertweedale]

(draft PR; I might add more commits)

"Minor" in the sense that it's mostly just moving definitions around. Definition or type changes are few, and trivial.

• Move ResourceSpec, EntityCommand and TempfileOnExit types, and their companion optics, from Purebred.Types.Mailcap to Purebred.System.Process This is a more appropriate home.

• Remove unused function handleIOException

• Move decodeLenient to new module Purebred.Types.String. This avoids the full import of Purebred.Types in Purebred.System.Process, which avoids an import cycle due to the moved definitions.

• Move toProcessConfigWithTemplate from Purebred.System.Process to Purebred.Types.Mailcap. It becomes a non-exported helper function.

• Move Purebred.Storage.Mail.mkConfig to Purebred.Types.Mailcap.mailcapHandlerToEntityCommand. It becomes a pure function (previously had a redundant use of pure and a redundant constraints). The implementation is otherwise the same. Refactor call sites.

• Refactor openCommand' to use mailcapHandlerToEntityCommand.

[frasertweedale]

Alright, thanks. I'll merge it now and open new PRs for subsequent changes.