Fix libpcre2 bindings function pointers

[straight-shoota]

The function binding for pcre2_general_context_create receives two function pointers.
They're typed as void pointers instead of a proc type.
The problem is that at the call site the argument type of one of the callback functions is incorrectly declared as LibC::Int, but it should actually be LibC::SizeT. The function definition mentioned the expected callback type, but it also incorrectly used Int.
This puts it in the same error class as #13088.

It sometimes worked - including on all platforms where the original PCRE2 implementation was tested, so it wasn't noticed.

Crystal's lib bindings actually has special syntax for function pointers as detailed in https://crystal-lang.org/reference/1.7/syntax_and_semantics/c_bindings/callbacks.html

Function pointers can be expressed as Proc type. And when using proc literals at call site, you can even skip the argument types because they're inferred from the function binding.
So yay, with this patch the code is safer and more concise.