Change MathematicalProgram::x_initial_guess_ from Eigen::VectorXd to std::vector<double>

[hongkai-dai]

When we need to push new entries to an array, using std::vector is much faster than Eigen::Vector.

Currently MathematicalProgram::x_initial_guess_ has the type of Eigen::VectorXd, and we often call conservativeResize on this vector, which is slow. I think it is better to change its type to std::vector<double>. And the return type of its getter function will be changed from Eigen::VectorXd to Eigen::Map<const Eigen::VectorXd>

Eigen::Map<const Eigen::VectorXd> MathematicalProgram::x_initial_guess() const {
}

@jwnimmer-tri WDYT?

[jwnimmer-tri]

I think we should never pursue any performance improvement without first building a peer-reviewed, application-level benchmark that demonstrates the symptom.