Add Ice.compileSlice public method to Ice for Python

[pepone]

This is useful to dynamically compile Slice to Python, instead of loadSlice

https://github.com/zeroc-ice/ice/blob/54e6c6d0b9e9d5208b600da21c061b7aa05d2ed9/scripts/Util.py#L30-L40

loadSlice is problematic when you load the same file multiple times, as classes get redefined and it can result in marshall exceptions, because the isinstance checks fail. For example a instance create for the previous definition.

[bernardnormier]

We should document the difference between compileSlice and loadSlice, as it's not obvious from their names.

[pepone]

We decide not to add this new method, and to improve loadSlice see #4733