Build a map (statefully) as we process declarations

[dorchard]

I noticed some problems with building a constant expression map were coming through into CamFort. This seems to fix things now (makes the associated tests pass in CamFort)- the main idea is in genConstExpMap to find all the statements the declare parameters or have a constants, and then evaluate them one at a time, building a map from values to FRepr representations.

[raehik]

Thanks a ton for this! I had a feeling I didn't replace the analysis constant evaluation properly.