feat[venom]: optimize get_basic_block()

[charles-cooper]

What I did

How I did it

How to verify it

Commit message

`get_basic_block()` is a hotspot in venom (up to 35% of total
compilation time!). this optimizes `get_basic_block()`, on a large
contract near the 24kb limit this reduces time spent in venom from 3s to
1s (total time from 6s to 4s).

note on the same contract, time spent in the IRnode optimizer pipeline
is 2s - so time in venom is now smaller than time in legacy optimizer(!)

notes:
- refactor to use dict for basic_blocks
- clean up basic blocks API 
    hide basic blocks behind `get_basic_blocks()` iterator and
    `num_basic_blocks`.

Description for the changelog

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 71.87500% with 18 lines in your changes are missing coverage. Please review.

Project coverage is 88.01%. Comparing base (3af5390) to head (093c4f3).

❗ Current head 093c4f3 differs from pull request most recent head 81d7311. Consider uploading reports for the commit 81d7311 to get more accurate results

Files	Patch %	Lines
vyper/venom/function.py	76.47%	7 Missing and 1 partial ⚠️
vyper/venom/passes/simplify_cfg.py	46.15%	5 Missing and 2 partials ⚠️
vyper/venom/analysis/dominators.py	0.00%	1 Missing ⚠️
vyper/venom/ir_node_to_venom.py	66.66%	1 Missing ⚠️
vyper/venom/passes/normalization.py	50.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4002      +/-   ##
==========================================
- Coverage   90.86%   88.01%   -2.86%     
==========================================
  Files         103      103              
  Lines       14908    14905       -3     
  Branches     3304     3303       -1     
==========================================
- Hits        13546    13118     -428     
- Misses        941     1291     +350     
- Partials      421      496      +75     

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