Fix capture of same variable twice - closes #237

[filipsajdak]

In the current implementation, when the move from the last use happens to a variable captured, it will be captured twice: one by copy and one moved.

That means that the following code:

#include "boost/ut.hpp"
#include "html/html.h2"

using namespace boost::ut;
using namespace std::literals;

main: () -> int = {
    test("html::element") = :() = {
        e: html::element = ("tag");

        should("setting attribute") = :() = {
            mut(e$).set_attribute("name", "value");
            expect(eq(to_string(e$), std::string_view("<tag name=\"value\"></tag>")));
        };
    };
}

Will generate (skipping boilerplate):

[[nodiscard]] auto main() -> int{
    test("html::element") = [](){
        html::element e {"tag"}; 

        should("setting attribute") = [_0 = e, _1 = std::move(e)](){
            CPP2_UFCS(set_attribute, mut(_0), "name", "value");
            expect(eq(to_string(_1), std::string_view("<tag name=\"value\"></tag>")));
        };
    };
}

This change makes cppfront identify that variable is moved, capture it once by the move, and use it multiple times. That means that the above code will generate the following:

[[nodiscard]] auto main() -> int{
    test("html::element") = [](){
        html::element e {"tag"}; 

        should("setting attribute") = [_0 = std::move(e)](){
            CPP2_UFCS(set_attribute, mut(_0), "name", "value");
            expect(eq(to_string(_0), std::string_view("<tag name=\"value\"></tag>")));
        };
    };
}

[filipsajdak]

There is only one change in regression tests - in mixed-function-expression-with-repeated-capture.cpp file. The following code:

    y := "\n";
    std::ranges::for_each
        ( view, :(x:_) = { std::cout << y$ << x << y$; } );

Generates:

     auto y {"\n"}; 
     std::ranges::for_each
         ( view, [_0 = std::move(y)](auto const& x){std::cout << _0 << x << _0;});

instead of the following:

     auto y {"\n"}; 
     std::ranges::for_each
         ( view, [_0 = y, _1 = std::move(y)](auto const& x){std::cout << _0 << x << _1;});

[filipsajdak]

I have added overload of the print_to_string function that takes a pointer to std::string that will make it possible to eliminate an additional bunch of calls to printer.emit_to_string() that need to match earlier call to printer.emit_to_string(&str).

The new overload looks:

    void print_to_string(std::string* str, auto& i, auto... more) {
         printer.emit_to_string(str);
         emit(i, more...);
         printer.emit_to_string();
     };

[hsutter]

Looks good, thanks!