[WIP] Expose immediate base contracts and base constructors

[Xenomega]

This pull request aims to resolve #131 . It aims to provide the immediate base contracts of a given contract in the correct order of declaration via contract.immediate_inheritance. This contrasts to the existing provided contract.inheritance which provides a flattened, linearized base contract list (all ancestors in the inheritance chain, immediate and not).

[Xenomega]

This pull request has now been refactored. It additionally adds support for both AST formats. It also now exposes explicit base constructor calls on both contract definitions and constructor definitions. (Meaning base constructors called on a contract's constructor definition, or those called via the use of parenthesis on the contract definition).

The example below has explicit constructor calls on the contract definition for B and D, and explicit constructor calls on the constructor definition for C and D.

contract E is B(1), C, D() {
    constructor() C(2) D() public {

    }
}

NOTE: Please verify legacy-ast heuristics are sufficient, as legacy-ast is known to present unreliable nodes at times. All tests to this point have yielded expected results.