Distribution definition in BLOG.

[lileicc]

In BLOG, we define some library module with the following statement

distribution Real Gaussian(Real mu, Real std) = Gaussian(mu, std);

It defines a Gaussian distribution, with two Real parameters, and a Real return value. Its implementation is the Gaussian class (which is blog.distrib.Gaussian).
Equivalently, we could also state

distribution Real Gaussian(Real mu, Real std) = blog.distrib.Gaussian(mu, std);

Related to #136 .

[cberzan]

👍 This would allow type-checking on distributions, and avoid bugs like #155 that currently lead to confusing error messages.