add machine readable ir_json format

[charles-cooper]

What I did

Add JSON format for IR output. Makes it easier for tools to integrate our IR.

How I did it

Map LLL nodes with no args to JSON number or string, LLL nodes with args map to {"instruction": *args}.

How to verify it

Try it out

Description for the changelog

Add machine-readable JSON format for IR output

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[codecov-commenter]

Codecov Report

Merging #2510 (7a2150b) into master (cec2c53) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2510      +/-   ##
==========================================
+ Coverage   84.63%   84.67%   +0.04%     
==========================================
  Files          94       94              
  Lines        9273     9281       +8     
  Branches     2093     2095       +2     
==========================================
+ Hits         7848     7859      +11     
+ Misses        940      938       -2     
+ Partials      485      484       -1     

Impacted Files	Coverage Δ
vyper/compiler/__init__.py	85.71% <ø> (ø)
vyper/cli/vyper_compile.py	74.16% <100.00%> (ø)
vyper/compiler/output.py	97.31% <100.00%> (+0.15%)	⬆️
vyper/builtin_functions/functions.py	89.32% <0.00%> (+0.34%)	⬆️

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