improve array_splice inference

[orklah]

This should fix #5678

I mainly used array_slice return type provider to adapt it, this will be better than before but there is definitely some improvements to be made:

1. I'm not sure what this does: https://github.com/vimeo/psalm/compare/master...orklah:array_splice?expand=1#diff-5953e521e3d01187b486b9c7a2e7e57ca6c4aa65886e4825c1bc765de64029cbR50
2. The fourth param should be used to infer the return type. Right now I'm only using the first one.
3. There is a reference for the first param that should be treated the same way
4. I only return list<int>, list<string> or list<mixed> to avoid making a really long list. I searched for a method that would widen a type to standard scalars (for example 'foo'|'bar' would become string) but I couldn't find it

[muglug]

Thanks!