remove "skippable_macros" from preprocessor

The preprocessor no longer has the concept of skippable_macros and now
always skips any line that starts with "db" or "dw" because neither of
these should be treated as macros.

fixes #178
remove ItemFragment from preprocessor

It's not required.

see #178
Merge pull request #179 from kanzure/remove-preprocessor-skippable-macros

Remove preprocessor skippable macros