Pull LZW decompression code out into a omgpdf-stream.js.
This code is currently unused since pdf.js's stream code
is handling this for now.  One day it would be nice to
have our own, along with code for compression.
Generalize dict.get_type_checked as dict.get_checked.
Comment out obj_type, it's a bit ugly and should be
better to use obj_is_XXX() directly.