[RFC]: Add C implementations to truncated normal distribution functions (tracking issue)

This RFC proposes adding C implementations to all truncated normal distribution functions in `@stdlib/stats/base/dists/truncated-normal` .

Start by reviewing the truncated normal distribution package at @stdlib/stats/base/dists/truncated-normal and its existing functions. Determine the full set of truncated normal distribution functions covered by the RFC and inspect any related implementation guidance in the package. Done means C implementations have been added for all functions in that package.

索引モデルが issue の本文から書いたものです。