add 512 byte RAM for use as sector buffer to get rid of
sector_buffer being inferred as 4096 1-bit registers, and so
hopefully decongest the design.
Issue #31.
first cut at using block ram for sector buffer.
Writing to sectors will definitely not work yet.  Issue #31.
fix compile errors with previous commit. Issue #31.
fix sector_offset slicing for sector buffer address. Issue #31
make reading clock synchronous. Issue #31.
adjust tests.