add factory decorator

[charles-cooper]

this will make it easier to deploy factory contracts and verify them on etherscan

also implements a portion of #2891

What I did

add factory decorator to init functions

How I did it

add it to function type metadata; in the codegen, if init function is marked as factory, wrap it in another deploy operation. this generates the correct initcode.

How to verify it

check code for this contract

FOO: immutable(uint256)

@external
@factory
def __init__(arg: uint256):
    FOO = arg

@external
def foo() -> uint256:
    return FOO

tests tbd

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

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[codecov-commenter]

Codecov Report

Merging #2918 (e8a50ab) into master (58a5ae5) will decrease coverage by 0.06%.
The diff coverage is 11.11%.

❗ Current head e8a50ab differs from pull request most recent head 5a73748. Consider uploading reports for the commit 5a73748 to get more accurate results

@@            Coverage Diff             @@
##           master    #2918      +/-   ##
==========================================
- Coverage   88.16%   88.09%   -0.07%     
==========================================
  Files          95       95              
  Lines       10579    10588       +9     
  Branches     2533     2536       +3     
==========================================
+ Hits         9327     9328       +1     
- Misses        792      798       +6     
- Partials      460      462       +2     

Impacted Files	Coverage Δ
vyper/codegen/module.py	91.81% <0.00%> (-2.58%)	⬇️
vyper/semantics/types/function.py	85.55% <16.66%> (-1.57%)	⬇️

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[charles-cooper]

meeting notes: punt until after v0.3.4

[charles-cooper]

closing because this is probably better left to tooling, especially given that users probably want to deploy factories with custom prefixes - see #2930

[charles-cooper]

the initcode optimization portion of this PR was moved to #2933.

[charles-cooper]

meeting notes: add -f bytecode_blueprint output format