feat[lang]: add inline decorator

[charles-cooper]

What I did

draft. depends #4478

How I did it

How to verify it

use @inline decorator on an internal function and compile it with -O codesize

Commit message

Commit message for the final, squashed PR. (Optional, but reviewers will appreciate it! Please see our commit message style guide for what we would ideally like to see in a commit message.)

Description for the changelog

Cute Animal Picture

[codecov]

Codecov Report

Attention: Patch coverage is 60.86957% with 9 lines in your changes missing coverage. Please review.

Project coverage is 91.96%. Comparing base (a466dc8) to head (decccc1).

Files with missing lines	Patch %	Lines
vyper/semantics/types/function.py	46.15%	5 Missing and 2 partials ⚠️
vyper/venom/passes/function_inliner.py	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4481      +/-   ##
==========================================
- Coverage   91.99%   91.96%   -0.03%     
==========================================
  Files         120      120              
  Lines       17292    17305      +13     
  Branches     2930     2933       +3     
==========================================
+ Hits        15908    15915       +7     
- Misses        963      967       +4     
- Partials      421      423       +2     

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