Refactor ABI generation

[iamdefinitelyahuman]

What I did

Refactor the ABI generation logic to happen within the new type objects.

How I did it

• Added the _metadata attribute to all node objects. This is an all-purpose dict for annotating nodes with information that doesn't strictly belong in the AST. For now it's being used very sparingly, but this is the approach I'm envisioning for passing type-check info into LLL gen. So good to discuss it now while there aren't many places to change it, if desired.
• Add to_abi_dict methods to ContractFunction, Event and InterfacePrimitive objects. The logic is largely copied from the old EventSignature and FunctionSignature objects.
• Generate an InterfacePrimitive from the top-level node during type checking, and add it to the _metadata so it can be used to build the ABI.
• Minor bugfixes around canonical types and signatures
• Remove the old, now ded code in signatures/

How to verify it

Run the test suite. There is already extensive coverage around ABI generation.

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[codecov-io]

Codecov Report

Merging #2284 (ed13879) into master (f8782e0) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2284      +/-   ##
==========================================
+ Coverage   85.42%   85.49%   +0.06%     
==========================================
  Files          84       84              
  Lines        8586     8583       -3     
  Branches     2078     2076       -2     
==========================================
+ Hits         7335     7338       +3     
+ Misses        748      745       -3     
+ Partials      503      500       -3     

Impacted Files	Coverage Δ
vyper/signatures/function_signature.py	77.71% <ø> (-3.70%)	⬇️
vyper/ast/nodes.py	93.39% <100.00%> (+0.01%)	⬆️
vyper/compiler/output.py	100.00% <100.00%> (ø)
vyper/context/types/function.py	85.64% <100.00%> (+2.40%)	⬆️
vyper/context/types/indexable/sequence.py	84.50% <100.00%> (+0.92%)	⬆️
vyper/context/types/meta/event.py	76.36% <100.00%> (+0.89%)	⬆️
vyper/context/types/meta/interface.py	81.13% <100.00%> (+3.68%)	⬆️
vyper/context/validation/module.py	87.57% <100.00%> (+0.14%)	⬆️
vyper/signatures/event_signature.py	94.87% <100.00%> (-0.88%)	⬇️
vyper/signatures/sig_utils.py	100.00% <100.00%> (ø)
... and 1 more

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