that supports writing to flash memory, and it should be used for
all cases where access to flash memory is desirable.
- REQUESTS is the vector (see vec.h) of memory_write_request.
+ REQUESTS is the vector of memory_write_request.
PRESERVE_FLASH_P indicates what to do with blocks which must be
erased, but not completely rewritten.
PROGRESS_CB is a function that will be periodically called to provide