[spec] Runtime format of element and data segments

[binji]

This is copied and modified from this closed PR:

#19

[rossberg]

This looks correct. However, wouldn't it be more appropriate if an eleminst held a vector of references? The expressions from the segment should be evaluated only once, during instantiation of the module.

To model this, instead of a data segment, allocelem would take the vector of refs as parameter, and the evaluation would occur in the module instantiation rule, analogous to the init values for globals.

For symmetry, allocdata should take a plain byte vector as parameter as well.

[binji]

Yes, that makes sense. I'll update the PR