Change syntax for size-assertion slices. This is an incompatible change.

Simplify the syntax by replacing the [=nnn] syntax with just [nnn].  Slices
that do not specify a size are written [0+nnn]; the old backup tool has
been doing this already since about v0.8.  Before that point, [nnn] was a
shorthand for [0+nnn], and will be mis-interpreted by modern tools.