[WIP] Package invariants

[jcp19]

Changelog:

• we introduce the notion of package invariants to describe the contents of global state (we distinguish between non-duplicable and duplicable invariants - more details to follow soon). We introduce the statement openDupPkgInv to open the duplicable invariants of the current package if initialization is guaranteed to be finished, and we introduce all necessary proof obligations for the init functions and main function.
• we introduce the notion of functions that may be called during initialization (marked with mayInit) to make sure we never open invariants that might not hold yet - more details on this soon
• we deprecate importRequires and initEnsures clauses to allow for modularly checking a package. We also deprecate the --lazyImports flag, as it is no longer necessary.
• we make the Desugarer sequential again to fix the regression reported in Regression same_package/import-fail01/main fails non-deterministically #762