fix: reject booleans in isinstance int checks

[dinxsh]

Fixes #30

Python's bool is a subclass of int, so isinstance(True, int) returns True. In cryptographic code this is dangerous - a boolean passed where an integer is expected can produce incorrect results silently.

Changes:

• Adds is_integer() utility in py_ecc/utils.py that explicitly rejects booleans
• Updates field arithmetic checks in field_elements.py, optimized_field_elements.py, ciphersuites.py to use it
• Adds tests verifying booleans are rejected and integers accepted