fix zip archives paths and add zip files test

- create_zip from a subdirectory wrote entry names with a leading
  '/', so the mounted archive looked empty
- memory_ostream did not support tellp(), so create_zip into a memory
  device wrote invalid offsets (-1) to the archive
- ZipFileDevice added missing parent directories for the first entry
  only
- ZipFileDevice::exists returned false for the archive root
- MemoryDevice::mkdirs recreated existing directories, dropping their
  content (mkdir("a/b") cleared "a"); overwriting a file duplicated its
  name in the directory listing

Added dev/tests/zip_files.lua: builds archives with file.create_zip
from memory device root, subdirectories and the real filesystem,
mounts them and checks reading, listing and read-only access.
memory_filesystem.lua got checks for the MemoryDevice issues.

Closes #544

dev/tests/memory_filesystem.lua

@@ -41,6 +41,17 @@ assert(entries[1] == "memtest:dir/subdir/a.txt")
 assert(entries[2] == "memtest:dir/subdir/b.txt")
 assert(entries[3] == "memtest:dir/subdir/other")
 
+debug.log("create sibling directory")
+file.mkdir("memtest:dir/subdir/sibling")
+assert(file.isfile("memtest:dir/subdir/a.txt"))
+assert(file.isdir("memtest:dir/subdir/other"))
+asserts.equals(4, #file.list("memtest:dir/subdir"))
+
+debug.log("overwrite file")
+file.write("memtest:dir/subdir/a.txt", "overwritten")
+asserts.equals("overwritten", file.read("memtest:dir/subdir/a.txt"))
+asserts.equals(4, #file.list("memtest:dir/subdir"))
+
 debug.log("remove tree")
 file.remove_tree("memtest:dir")
 assert(not file.isdir("memtest:dir"))