Drop mimeType and ctime from the manifest format (closes #235)

Nothing goes in the 1.0 manifest that 1.0 does not read or write, and
mfer never wrote either field. mf.proto now reserves the numbers 301 and
303 and both names in MFFilePath, and mf.pb.go is regenerated. The
decoded-size check counts only file entries, hashes and timestamps, and
counts a file entry, 16 bytes smaller without the two fields, at 160
bytes. export prints no ctime, and FileEntry loses the Ctime nothing
set. FORMAT.md and the README no longer describe either field.

The test of entries that decode too large gives each entry a second,
empty hash: an entry with one hash and a modification time can no
longer reach 8 times its size.

Model: opus-5-5

mfer/mf.proto.sha256

@@ -1 +1 @@
-1668b2f6b8639a9213873474e58dbacb3a4ef77cc27c42253cad8dd076e5f206  mf.proto
+16a2e6b39986d02aa1c7c01dc6b97c9d4c30db7a9d000056bd5985a3fa7c8089  mf.proto