Module etcetera

Available on doc or test only.

etcetera modifications

(↑) This is derived work from the etcetera crate, including the following modifications:

• make the trait dyn compatible.
• rename AppStrategyArgs to AppData.
• unify all Xdg structs into DirXdg.
• unify all Apple structs into DirApple.
• unify all Windows structs into DirWindows.
• unify BaseStrategy and AppStrategy into DirEnv.
• replace Result with Option, remove HomeDirError.
• validate AppData construction, made fields private.
• make Apple bundle ID match the directories crate.
• replace dashes with underscores in unixy_name.
• remove all deprecated windows-specific code.
• remove all unneeded dependencies.
• remove helper macros.
• remove all unsafe.