Instead of mostly heuristics, actually execute the init() instruction in
an interpreter to calculate initializers for globals. This is far more
flexible and extensible, and gives the option of extending the
interpreter to other code and make it a partial evaluator.
This should make it much easier (in the future) to use the standard
library when unused functions contain unimplemented features. But more
importantly, it makes later passes better and makes compiling faster by
not having to scan dead code.
Previously, mostly the types from the ssa package were used directly
with the types from analysis.go as an overlay. This commit uses these
types everywhere and renames a few things here and there to make things
clearer.
