fix corner cases of "modint" when mod = 1

[mizar]

fixes #110

This pull request includes changes to the src/modint.rs file to address corner cases when the modulus is set to 1. The most important changes involve modifying the initialization logic for ModInt and adding tests to ensure the correctness of the implementation.

Modifications to initialization logic:

• src/modint.rs (pub trait ModIntBase): Changed the initialization value in the pow method to handle the case when the modulus is 1.
• src/modint.rs (macro_rules! impl_folding): Updated the Product implementation for StaticModInt and DynamicModInt to handle the modulus being 1.

Addition of tests:

• src/modint.rs (mod tests): Added a new test mod1_corner_case to check the behavior of ModInt when the modulus is set to 1, including edge cases for the product of an empty iterator and the power of zero.

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This PR is extracted from the content of #112 .

fix issue:

• Corner cases of "modint" when mod = 1 #110

related Issue/PR:

• mul_mod for 2^31 < m < 2^32 atcoder/ac-library#149
• fix #149: improve barret algorithm atcoder/ac-library#163
• ModInt 2^31 <= m < 2^32 for addition/subtruction atcoder/ac-library#164
• mul_mod for 2^31 < m < 2^32 #111
• u32 modulo fix #112
• Support modint under m = 1 #123

[qryxip]

BTW, why not actually link to #110?

[mizar]

BTW, why not actually link to #110?

Thanks for pointing that out. I have corrected the first message and linked it.

[mizar]

CI was fixed in #141 and rebased to master branch.