Make matches()/exec() work with non-mut pcre

[omakoto]

It doesn't look like these methods need to take a mutable reference -- though I'm new to rust so I can't really tell for sure... Am I missing something? (It still builds fine and all tests still pass.)

[TonalidadeHidrica]

I agree that this does not to be mutable. It's sad that this repository has been abandoned for years...