Files and paths

Decided

D-0068, settled by the language’s author: part of spec §18. Older text cites it as 18.68.

Path, File and Directory, with no free functions, chosen by the user: every operation belongs to the type it is about, and a Path type rather than bare str gives one place for joining, splitting and cross-platform separators. A Path always holds /; \ is accepted and converted, since Windows accepts / and nothing else does \. File is move-only and closes on drop; whole-file read_text/write_text beside a streaming open. Failures are !-> with an IoError code rather than typed errors (§9.2). Not yet: a cstr for a C API from a Path (as_str plus the owned-cstr row of the to-do list), file metadata such as modification times, and memory-mapped files (§14.4)