replace "params" slot in nodes of intermediate representation

[felix winkelmann]

Currently the nodes in the intermediate representation collect various information in a list ("node params"). This is error-prone and probably inefficient. It should be replaced by something safer. Since there are quite a number of node types, defining record for each and every node will probably result in code-bloat, so I suggest either a vector or some syntax-only lightweight quasi-structure facility.

The params will be printed when intermediate stage results are shown via the -debug option, so a readable representation (or something that can be converted to a readable representation) is desirable.