add cbor length metadata

[charles-cooper]

so disassemblers/analyzers can reliably detect where the code ends and
the CBOR metadata starts

What I did

fix #3009

How I did it

How to verify it

compile a vyper contract and inspect last 13 bytes

Commit message

add binary encoded length to the end of the CBOR metadata so
disassemblers can reliably detect where code ends and metadata starts

Description for the changelog

add length to CBOR metadata so bytecode analysis can reliably demarcate it

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #3010 (1aff960) into master (f31f0ec) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3010   +/-   ##
=======================================
  Coverage   88.29%   88.29%           
=======================================
  Files          97       97           
  Lines       10919    10920    +1     
  Branches     2583     2583           
=======================================
+ Hits         9641     9642    +1     
  Misses        830      830           
  Partials      448      448           

Impacted Files	Coverage Δ
vyper/ir/compile_ir.py	93.71% <100.00%> (+0.01%)	⬆️

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