The .dex writer

This is the implementation of the .dex writer (.dex IR -> .dex image).
Dexter was also modified to use the writer to create a new image and
optionally write it to an output file (-o)

There are a number of TODOs which I plan to address in a follow up CL
(or an new iteration of this CL if possible)

Test: existing dexter tests

Change-Id: Id48d9cb18a8c7b147d0a8a4106f1a7305cf2d943