Propagate extended code positions to more tactics

[strub]

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.

[fdupress]

It might be worth adding a quick note that the changes also include the ability to use a generalised codepos for wp, sp and some other interesting tactics.

[strub]

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