Add a *FAST* dictionary header reader.

It's still unused as of this change but the next change will use it

As a reference point, generating the metadata for Bayo takes
3'02" on my machine with the info command; it's down to 16" if
made to use this instead. The gains increases with the number
of dictionaries obviously.

Change-Id: I0eeea2d8f81bb74b0d1570af658e91b56f7c2b79