Bind _str_type to locals in get/set/delitem
Fix api_semidbm import names
Add fileformat spec
Switch db file to binary format
Add header info to binary file format

Only verifying major versions.  Minor versions
should be compatible.
Remove irrelevant test helper method
Merge branch 'binary-format'

This branch contains the work to switch from
a pure ASCII format to a binary format.  This brings
performance increases and simplified logic for loading
a db file.

I've also added version info to the file format, to help
determine if this a file we can load.

This new format is documented in the sphinx docs.

* binary-format:
  Remove irrelevant test helper method
  Add header info to binary file format
  Switch db file to binary format
  Add fileformat spec
  Fix api_semidbm import names