Replace `std::path::Prefix::Server` with `std::path::Prefix::Segment`

[egkoppel]

Exact name TBD - currently either Dir or Segment?

Servers can no longer be directly accessed with open calls, so having a path format to access a server is pointless

Standard segmented directories now exist in popcorn, so it makes sense to replace the now unused syntax to access segmented directories. A path of the form d:path/to/x should attempt to use a handle at fs.dir.d and then open the path path/to/x from it. No segment prefix should default to the cwd segment. Getters for typical segments (cwd, config, data, tmp, home-*?¹) should probably be added to std::os::popcorn::fs::DirExt. Thought should be given as to what executables with full FS access do to access files relative to root (root: pseduo-segment?), and what happens if an absolute path is requested from a segment.

Footnotes

1. Should home-* directories ever actually be passed directly to processes or for storing user data should everything go via the file-picker server (and if so should file-picker server methods be exposed in FileExt and DirExt as constructors) ↩