primop: add readFileType, optimize readDir

Allows checking directory entry type of a single file/directory.

This was added to optimize the use of `builtins.readDir` on some
filesystems and operating systems which cannot detect this information
using POSIX's `readdir`.

Previously `builtins.readDir` would eagerly use system calls to lookup
these filetypes using other interfaces; this change makes these
operations lazy in the attribute values for each file with application
of `builtins.readFileType`.

tests/lang/eval-okay-readFileType.exp

@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }