backends/chert/chert_table.cc: Document situations in which the
cursor can be left pointing before DIR_START.
backends/chert/chert_check.cc: Add check that dir_end is odd.
backends/chert/chert_table.cc: In enter_key(), we know the insertion
point will be the position after the current cursor position, so
there's no need to call find_in_block() to locate it.
backends/chert/chert_table.cc: Add more assertions that c has
suitable values.
backends/chert/chert_table.cc: Fix assertion to allow for c being
DIR_START - D2 in leaf blocks.