Implement fact export for manually-written and inferred function contracts

[yuxincs]

This PR implements the fact export for manually-written and inferred function contracts. Specifically it enhances the functioncontracts sub-analyzer, which imports contracts from upstream packages, parses/infers the contracts for the local package, combine the contract map and return it. It now also exports the contracts for the exported functions in the local package to be consumed by its downstream packages.

Tests are also added to ensure the fact export and import logic works.