[w1tcherrr]

Adds Package.RenameFile to move an entry without reading its data and adding it again.

Also fixes what Valve does:

• Paths with an empty file name or extension, like .hidden or trailing., or a null character, now throw instead of writing a broken VPK.
• Paths are lowercased like Valve does, because the game can't find entries with uppercase letters. On a package that isn't optimized with OrdinalIgnoreCase, FindEntry needs the lowercased path to find an added file.
• Leading and trailing slashes are trimmed from added and renamed paths, the same as FindEntry already does.
• Renaming onto a path that only differs in case from an existing entry throws.
• Write only truncates an existing file after writing, so a package that can't be written leaves it untouched.

And a few other fixes:

• Adding, removing, renaming or writing after Dispose throws instead of truncating chunk files.
• Read throws on a package that files were added to, instead of dropping them.
• Malformed archive MD5 section sizes and entry lengths throw before allocating, instead of allocating gigabytes.
• An other MD5 section that isn't 48 bytes is skipped, so the signature is read from the right offset.
• IsSigned tells unsigned packages apart, since IsSignatureValid returns true for them.
• VerifyHashes reports tree and archive section checksum mismatches instead of throwing when the whole file checksum matches, because many of Valve's map paks have garbage there. It also returns for version 1 packages, which have no checksums. It now takes a progress reporter like the other verify methods.
• WriteChunkSize defaults to 100 MiB and a new chunk file only starts once the current one is larger than it, like Valve's workshop uploader.