feat[lang]: add ABIBuffer type

[charles-cooper]

What I did

bypasses abi encoding of return types

How I did it

How to verify it

check the following code

def foo() -> ABIBuffer[32]:
    return x"abcdef"  # raw abi encoded data

@external
def bar() -> ABIBuffer[32]:
    return x"abcdef01"

@external
def baz() -> ABIBuffer[128]:
    res: Bytes[128] = raw_call(self, method_id("bar()"), is_delegate_call=True, max_outsize=128)
    return convert(res, ABIBuffer[128])

Commit message

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Description for the changelog

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[codecov]

Codecov Report

Attention: Patch coverage is 97.72727% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.65%. Comparing base (2d515d3) to head (a10e7e6).
Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
vyper/builtins/_convert.py	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4561      +/-   ##
==========================================
- Coverage   92.70%   92.65%   -0.06%     
==========================================
  Files         123      122       -1     
  Lines       17526    17565      +39     
  Branches     2971     2984      +13     
==========================================
+ Hits        16248    16275      +27     
- Misses        879      892      +13     
+ Partials      399      398       -1     

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[cyberthirst]

event FooEvent:
    a: ReturnBuffer[100]

interface Foo:
    def foo() -> ReturnBuffer[100]: view

@external
def foo(target: Foo):
    log FooEvent(a=staticcall target.foo())

yields

vyper.exceptions.StructureException: not a valid tuple member: ReturnBuffer[100]

  contract "tests/custom/test.vy:10", function "foo", line 10:4 
        9 def foo(target: Foo):
  ---> 10     log FooEvent(a=staticcall target.foo())
  ------------^

i don't think that "not a valid tuple member" is a very clear ex

[cyberthirst]

note that the fact, that we don't allow conversion into Bytes severely limits how the type can be used

the type doesn't support slice, concat, len, or abi_decode, it can be converted to Bytes but can't be converted back

thus it's basically impossible to do post-processing on it