Better define link references

[gnidan]

Update of #70

Fixes https://github.com/ethpm/ethpm-spec/issues/69

What was wrong

In the v1 version of the spec, link references were only defined as the locations within a block of bytecode in which an __Thing____ placeholder was found. This is problematic because:

1. It is imprecise, requiring regex based detection.
2. There are edge cases which cannot be handled once you start dealing with link values of varying lengths.

How was it fixed

Now, there is a Bytecode object which defines three fields.

• bytecode: the actual bytecode itself
• link_references: the locations in the bytecode that require linking
• link_dependencies: the values that the bytecode should be linked with.

Link references are further defined in a Link Reference object which contains three fields:

• offsets: the start positions of the link reference
• length: for how long the link reference is
• name: for grouping of multiple link references which are expected to be linked with the same value. (optional)

All bytecode and runtime_bytecode fields have been replaced to use the Bytecode object definition instead.

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[gnidan]

@pipermerriam @mkosowsk please see recent commits and let me know if you have any other thoughts. Thanks!

[mkosowsk]

@gnidan most recent commits pass smell test for me altho I don't have cycles to read for depth at the moment. LGTM 👍🏻

[gnidan]

Merging. Thanks for the feedback!