fix: allow ARC4 struct from_bytes with undersized backing bytes

[mitre88]

Summary

AVM construction of an ARC-4 struct from a short byte string does not fail immediately. It only errors when an operation reads past the backing bytes.

Struct.from_bytes(b\"\\x00\\x00\") on a type with a large static array used to raise input string is not long enough to be decoded immediately. It now stores the bytes, lets in-range fields (e.g. length) decode, and errors on out-of-range access (arr[0]).

Test plan

• pytest tests/arc4/test_struct.py — 15 passed (includes new short-input case)
• pytest tests/arc4/test_{struct,uintn,bool,string}.py

Fixes #45