Defer writes until a full sector is buffered

[avsa242]

Right now even very small writes to a sector write the sector in full to the card, which blocks for a long time. Add logic to wait until the sector buffer is full, or is explicitly flushed by the user.

• add a separate metadata buffer
• find a way to eliminate the need to read in the FAT every time fseek() is called, to reduce unnecessary SD access; maybe set a flag indicating what kind of data is currently in the metadata buffer, and if it's the correct FAT sector, then do nothing
• add a method to 'queue' data
• add a mechanism for explicitly flushing the write buffer
• rewrite fwrite() to wait until sector buffer is full