Improve comments, remove dead code, and increase readability

A series of small fixes to readability in comments and logic in
bindings_composer.
Improve reporting of syntax errors in interpolations in binder_config

Before this fix, the information was Syntax error line, pos, but no
file. This fix uses the file (or a reference to the ruby file that
defined the defaults), and sets that in a syntax error exception if file
information is not already set.