Consider lazily expanding entities

[ianprime0509]

Depends on #30

#30 will offer some obvious efficiency improvements for Reader (reducing the amount of copying necessary). A further improvement which warrants its own issue is the potential to avoid eagerly expanding entities in text content (as described in #29) and instead offering functions on Event to expand raw content on demand. This would avoid such processing for events where the user doesn't care about the expanded content.

This has quite a bit of overlap with the proposed design of #29, and would probably mean that Reader and Parser would use the same Event structure and common expansion functions.

[ianprime0509]

Implemented in the design of #36.