perf: avoid redundant validation in copy() and __iter__

[P403n1x87]

BaseInstr.copy() previously reconstructed instructions via __init__, triggering the full _set → _check_arg validation chain on every copy even though the source instruction is already known to be valid. Replace with object.__new__ + direct slot assignment to bypass validation entirely.

_BaseBytecodeList.__iter__ ran _check_instr on every yielded item, but Bytecode.__iter__ (which calls this via super()) was already doing the same check — a redundant double-validation on every iteration.

The script used for validating the change does a round-trip decompile/recompile for 1 second:

from time import perf_counter_ns as time

def test_roundtrip(code_object):
    end = time() + 1e9  # 1 second
    c = 0
    while time() < end:
        c += 1
        Bytecode.from_code(code_object).to_code()
    print(f"{code_object.co_name!r}: {c} round-trips/s")

Profiling shows the following own CPU time figures:

Function	Before	After	Relative change
Instr._check_arg	7.85%	3.13%	−4.72%
BaseInstr._set	2.57%	1.37%	−1.20%
_BaseBytecodeList.__iter__	2.18%	0.0%	-2.18%

Throughput (number of iteration within the 1 second duration) goes from ~90 to ~105.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.17%. Comparing base (66584dc) to head (40f07b3).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #193   +/-   ##
=======================================
  Coverage   95.17%   95.17%           
=======================================
  Files           7        7           
  Lines        2032     2032           
  Branches      449      448    -1     
=======================================
  Hits         1934     1934           
  Misses         54       54           
  Partials       44       44           

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[MatthieuDartiailh]

LGTM I will merge when you mark this as ready.

[P403n1x87]

@MatthieuDartiailh open for review 🙏