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Second attempt to resolve build failures for debian package.

Restructures tests to move prop tests into a prop_tests module and then verified that all combinations of feature flags build successfully.

• bump dependencies, namely quickcheck and rand
• resolve running tests without default features

Added List::position_entry(id) API to retrieve the index of a list entry.

New contributors: @TTWNO

Adds an API to MerkleReg for iterating over all nodes

Two new API's have been added to MerkleReg for traversing the Merkle DAG:

MerkleReg::children(hash: Hash)
MerkleReg::parents(hash: Hash)