Add `needs_bytecode_linking` to `LinkableContract`

[njgheorghita]

What was wrong?

Handling link references strategy was insufficient for pre-linked contract factories.

This is needed for the linker strategy introduced to pytest-ethereum [here]().

How was it fixed?

Replaced Contract.has_linkable_bytecode and Contract.is_bytecode_linked with Contract.needs_bytecode_linking that will automatically detect if a contract's link references are already linked.

some thoughts on possible improvements...

• option to validate link-refs against an address rather than simply asserting they are not 0s
• support partially pre-linked contract factories (right now every link-ref has to be unlinked, otherwise we assume that the factory is entirely linked.

Cute Animal Picture

[pipermerriam]

support partially pre-linked contract factories (right now every link-ref has to be unlinked, otherwise we assume that the factory is entirely linked.

This one seems necessary (but not necessarily for this PR)