make info about all declarations & definitions accessible to each rule

[duaraghav8]

The linter should scan all solidity code files for declarations & definitions of data, functions, contracts & libraries.
This "symbol table" should then be provided to all rules.

This structure will directly serve as the foundation for solving #11 & #79 (no-use-before-define rule) and allow a rule to access information outside the file on which the rule is being run.

As of this writing, Solium is file-aware. The goal is to make it codebase-aware.

[duaraghav8]

WHETHER WE SHOULD IMPLEMENT THIS OR NOT IS STILL UNDECIDED

[duaraghav8]

we could take a solc-like approach:

solium.lint({
  "filename1": "code",
  "filename2": "code"
});