Allow limited mutability (changing value)

[cowtowncoder]

Although current tr13 structure is fundamentally immutable in some respects -- can not add entries for example -- it would be possible to add some forms of mutability.

And specifically, it should be possible to change value of an existing entry, iff length of value is not changed.

It might even be possible to allow deletion by some use of tombstones; although this is something application might be able to fake by regular changing of value, using some bit marker within value to indicate "has been deleted" condition.