🦟 Bug fix

Fixes #1478

Summary

This commit should reduce load times significantly as it caches the first parse of the spec so that it does not need to be reparsed during construction through static initiallization of a cache.

On my m4 macbook air this reduces the following benchmark:

#include <sdf/Model.hh>
#include <sdf/Root.hh>

void deserialize()
{
    std::string sdf = "<?xml version=\"1.0\" ?><sdf version='1.6'></sdf>";
    sdf::Root root;
    sdf::Errors errors = root.LoadSdfString(sdf);
    if (!root.Model())
    {
        return;
    }
    sdf::Model _model = *root.Model();
    return;
}

int main(int argc, char **argv)
{
    std::chrono::steady_clock::time_point begin = std::chrono::steady_clock::now();
    for (int i = 0; i < 100; i++)
    deserialize();
    std::chrono::steady_clock::time_point end = std::chrono::steady_clock::now();

    std::cout << "Whole function call took: " <<
        std::chrono::duration_cast<std::chrono::microseconds>(end - begin).count()
        << "[µs]" << std::endl;
}

from running for 300000µs to 3000µs.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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