Extend pash with pure command

[angelhof]

This allow users to wrap pure commands of their scripts without having to write annotations for them. It would make especially a lot of sense for functions.

[angelhof]

It could be used as follows:

pure -i in1 in2 in3 -o out1 out2 out3 -- comm args

where -i indicates the input files, -o indicates the output files, and everything after -- is the command to run.

This should be implemented as the identity (for when PaSh decides not to compile a fragment that contains it) and when PaSh sees it, it should know the inputs and outputs of that command.

[angelhof]

Subsumed by #453