mpt: change to variable-width Key and Val types#80
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Until now the client of pmem has not needed to know the size of the space used on the on-disk leaf file. Variable-sized keys and values is going to change that. Add tracking of the size of the disk data. Also be more explicit about the invariants guaranteed in transactions with respect to ReadDisk/WriteDisk, and test disks thoroughly in the random testing.
Allow variable-width Key and Val, so that applications are not locked into 32-byte blobs as the only possible format. Update disk format to support variable-width keys and values. The first write of a key reserves space for the number of bytes in the associated value. Future updates can overwrite that space as long as the value does not grow in size. If the value does get larger, then the existing space is abandoned (leaked) and new space is allocated. That strategy should be fine for now, since all of our applications have fixed-size values anyway. Also create and use new testdata/treehash.txt, test vectors for tree creation and snapshotting.
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Key and Val are now slices instead of fixed-width [32]byte values.
Client code will need updating, especially code that expects key1 := key2 to make a copy of the key.