When ZODB is not around, we use a custom exception class for BTree-
internal conflicts (as opposed to those raised by the storage).

No docs part yet.

Don't need ZODB / functest to test string comparison stuff.

Remove module-scope imports of MUTs, etc.

Mark (temporarily) tests which use DB, etc. as functests.

Conditionally import the BTreesConflictError exception type from
ZODB.POSError into BTrees.Interfaces;  if missing, derive from ValueError.

Note that this is basically what the C code does already, so make
it use the one from BTrees.Interfaces directly.

Drop testmodule-scope imports of MUTs.