Add API to overwrite existing pass from plugin

[clairexen]

No description provided.

[povik]

Code looks good but the Python binding interacts badly with it:

[ 99%] Building kernel/python_wrappers.o
kernel/python_wrappers.cc:417:16: error: 'replace_existing_pass' marked 'override' but does not override any member functions
                virtual bool replace_existing_pass() override;
                             ^
kernel/python_wrappers.cc:417:16: warning: 'YOSYS_PYTHON::Pass::replace_existing_pass' hides overloaded virtual function [-Woverloaded-virtual]
./kernel/register.h:73:15: note: hidden overloaded virtual function 'Yosys::Pass::replace_existing_pass' declared here: different qualifiers ('const' vs unqualified)
        virtual bool replace_existing_pass() const { return false; }
                     ^
1 warning and 1 error generated.

[povik]

It's not pretty but I pushed a fix to the binding generator to handle this.

The below loads:

import libyosys as ys

class StatPass(ys.Pass):
    def __init__(self):
        super().__init__("stat", "")

    def py_replace_existing_pass(self):
        return True

    def py_execute(self, args, design):
        pass

p = StatPass()

[mmicko]

Wrote directly to @povik but just to confirm that I have compared generated file before and after, and not just that it fixes this one problematic line but also improves const correctness all over the place in python API wrapper for other calls.