Use valtrees as the type-system representation for constant values
This is not quite ready yet, there are still some problems with pretty printing and symbol mangling and `deref_const` seems to not work correctly in all cases.
Mainly opening now for a perf-run (which should be good to go, despite the still existing problems).
r? `@oli-obk`
cc `@lcnr` `@RalfJung`