Lazy substitution of macros in .bib files

[Andrey Golovizin]

BibTeX's .bib files can contain string macros defined by @STRING commands. Pybtex currently substitutes macros with their string values during parsing of .bib files. This poses a couple of problems:

1. pybtex-convert is unable to preserve BibTeX macros in the output.
2. BibTeX's .bst styles can define their own macros. This means that the result of parsing a .bib file depends on a bibliography style used. This complicates things. For example, there is no way to pass a BibliographyData object to the Engine. Instead of that the Engine has to parse the .bib file itself to be able to apply the correct macros defined by the bibliography style.