fix: uninitialized immutable values

[charles-cooper]

What I did

fix #3101

How I did it

ensure msize is initialized past end of immutables section before entering the constructor.

How to verify it

the provided test cases fail on master (97ff017) but not on this branch.

Commit message

immutable variables can be read before assignment in constructor code,
and their memory location is accessed, but that memory might not yet
be initialized. prior to this commit, its value is not necessarily
`empty(type)` since memory could have been written to ephemerally. in
particular, `create_copy_of` (and its sister, `create_from_blueprint`)
use `msize` to determine a starting location for where to copy the
target bytecode into memory, while the immutables section start is
determined using the static memory allocator. in case that `msize` is
still less than the immutables section end, `create_copy_of` can write
to the immutables section, thereby resulting in reads from the
immutables section to return garbage.

this commit fixes the issue by issuing an `iload
<immutables_section_end> - 32` before executing any initcode, which
forces `msize` to be initialized past the end of the immutables section
(and therefore, accessing an immutable before it is initialized in the
constructor will produce the "expected" `empty()` value for the
immutable).

note that a corresponding `mload` is not required for runtime code,
because vyper requires all memory variables to be instantiated at the
declaration site, so there is no way that msize can produce a pointer to
an uninitialized memory variable.

Description for the changelog

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #3409 (6a67eed) into master (4f9f813) will increase coverage by 0.08%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##           master    #3409      +/-   ##
==========================================
+ Coverage   89.09%   89.17%   +0.08%     
==========================================
  Files          84       84              
  Lines       10763    10765       +2     
  Branches     2452     2453       +1     
==========================================
+ Hits         9589     9600      +11     
+ Misses        781      771      -10     
- Partials      393      394       +1     

Impacted Files	Coverage Δ
vyper/codegen/module.py	97.61% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes

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