chore: serialize ir_runtime output for vyper-serve

[tserg]

What I did

Fix #3075.

How I did it

Cast ir_runtime to str.

How to verify it

1. Run vyper-serve in a console.
2. Run curl -X POST localhost:8000/compile -H "Content-Type: application/json" -d '{"code": "\n\n# @version ^0.3.7\n\n@external\ndef foo():\n pass\n"}' in another console. The compiled code should be printed in console.

Commit message

chore: serialize ir_runtime output for vyper-serve

Description for the changelog

Serialize ir_runtime output for vyper-serve

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[codecov-commenter]

Codecov Report

Merging #3338 (c030d5f) into master (a4c2449) will decrease coverage by 0.37%.
The diff coverage is 0.00%.

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@@            Coverage Diff             @@
##           master    #3338      +/-   ##
==========================================
- Coverage   88.93%   88.56%   -0.37%     
==========================================
  Files          84       84              
  Lines       10606    10607       +1     
  Branches     2215     2215              
==========================================
- Hits         9432     9394      -38     
- Misses        768      804      +36     
- Partials      406      409       +3     

Impacted Files	Coverage Δ
vyper/cli/vyper_serve.py	0.00% <0.00%> (ø)

... and 5 files with indirect coverage changes

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