ENH: Speed up truncnorm's ppf()and rvs() methods for scipygh-11299.

Addresses scipygh-11299.
Adds an rvs() method to truncnorm modelled on geninvgauss_gen.rvs().
Replaced np.vectorize'd truncnorm functions with _scalar versions.
Any broadcasting is handling inside the distributions methods.
This enables generating many ppf values for the same shape a.b parameters
in a single call, thus making the truncnorm delta overhead calculation
just once.