Use absl's locale-independent conversion functions in XPRESS MPSolver interface

[flomnes]

Using such functions as std::stoi, std::stod, etc. caused us a lot of trouble because these functions are locale dependent, as illustrated below

#include <iostream>
#include <locale>
#include <string>

void convert(std::string locale)
{
    std::string value = "9.81";
    if (std::setlocale(LC_ALL, locale.c_str()))
    {
        double converted = std::stod(value);
        std::cout << "locale " << locale << " converted to " << converted << std::endl;
    }
    else
    {
        std::cout << "locale set failed for " << locale << std::endl;
    }
}
int main()
{
    convert("C");
    convert("fr_FR.utf8");
    // OUTPUT
    // locale C converted to 9.81
    // locale fr_FR.utf8 converted to 9
}

At first, we thought that using ScopedLocale would allow us to temporarily use the "C" locale, but this approach has multiple problems

1. It's not thread-safe. For example calling std::stod when the locale is being changed through std::setlocale is undefined behavior source.
2. C is hard, const char* pointers easily become dangling or invalid without the caller knowing

struct ScopedLocale {
  ScopedLocale() {
    oldLocale = std::setlocale(LC_NUMERIC, nullptr);
    auto newLocale = std::setlocale(LC_NUMERIC, "C");
    CHECK_EQ(std::string(newLocale), "C");
  }
  ~ScopedLocale() { std::setlocale(LC_NUMERIC, oldLocale); }

 private:
  const char* oldLocale;
};

We ran into an issue where the locale wasn't properly restored after using MPSolver::XpressInterface. So we decided to use absl's locale independent functions to perform that task.

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