Include imports in decls/fix consts and vars

[patrickt]

This makes three major changes:

• Imports are now included under the declarations tag in top-level file objects. They are still included under imports. This is more in line with the Go spec, which treats imports as declarations akin to consts and vars.
• var and const entities now contain a list of spec objects, which contain names, variables, and assigned values (if any). See the test cases for an example of this.
• Instead of being an array of arrays, declarations is now an array of objects. This simplifies structure access (in downstream Haskell, we had to parse declarations as a list of lists, then join it together, hence losing information about consts in the same toplevel clause, which broke iota propagation).

This fixes #38.

This is a backwards-incompatible change, and will be tagged as 0.2.0.

[codecov-io]

Current coverage is 56.72% (diff: 68.00%)

Merging #41 into master will increase coverage by 1.47%

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+ Hits            321        333    +12   
+ Misses          225        221     -4   
+ Partials         35         33     -2   

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[patrickt]

sigh