metadata: Fix encoding problem that led to mojibake when storing metadata strings that contained both unicode characters and a space (or '!') character.

The fix is to stop using w82s, which does not properly reconstitute unicode
strings. Instrad, use utf8 bytestring to get the [Word8] to base64. This
passes unicode through perfectly, including any invalid filesystem encoded
characters.

Note that toB64 / fromB64 are also used for creds and cipher
embedding. It would be unfortunate if this change broke those uses.

For cipher embedding, note that ciphers can contain arbitrary bytes (should
really be using ByteString.Char8 there). Testing indicated it's not safe to
use the new fromB64 there; I think that characters were incorrectly
combined.

For credpair embedding, the username or password could contain unicode.
Before, that unicode would fail to round-trip through the b64.
So, I guess this is not going to break any embedded creds that worked
before.

This bug may have affected some creds before, and if so,
this change will not fix old ones, but should fix new ones at least.

Types/MetaData.hs

@@ -224,6 +224,7 @@ data ModMeta
 	| DelMeta MetaField MetaValue
 	| SetMeta MetaField MetaValue -- removes any existing values
 	| MaybeSetMeta MetaField MetaValue -- when field has no existing value
+	deriving (Show)
 
 {- Applies a ModMeta, generating the new MetaData.
  - Note that the new MetaData does not include all the