Propagate extended code positions to more tactics

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Nearly all program tactics (`wp`, `sp`, ...) can now take extended
code positions as arguments.

Moreover, extended code positions have been extended s.t. they
can now target a assignment with a specific variable. The syntax is
`^x<-` where `x` is the program variable name.

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Ah, yes. I wanted to split them. I think I will simply add a note as you wrote.