math.big: make is_odd public and add test cases

[phoreverpheebs]

This PR makes the internal is_odd function public for the reason that external users of the library cannot directly access the digits array. Exposing this function allows a faster method of checking if an integer is odd meaning the caller doesn't have to do something cheeky like a.int() & 1 == 1.

Also a reinforcement on the method's safety by checking the length of the digit array. This is preferable over checking if the signum is 0, because the signum could be incorrect (for whatever reason).

🤖 Generated by Copilot at 29c4838

This pull request improves the testing and implementation of the is_odd function for big integers. It adds more test cases, makes the function public, and handles the zero input case. It also moves the function to the big_test.v file.

🤖 Generated by Copilot at 29c4838

• Make the is_odd function public and handle zero input (link)
• Move the is_odd function from vlib/math/big/integer.v to vlib/math/big/big_test.v (link)
• Add a new constant is_odd_test_data with various test cases for the is_odd function (link)
• Add a new test function test_is_odd that asserts the correct output of the is_odd function for each test case (link)