Implementing default trait for distribution types

[saresend]

Hey! I wanted to ask if you'd be open to merging a change that adds default implementations for various distributions? I've been using your crate and would like to implement default so that types that wrap the raw distributions can be made a bit more ergonomic for things like examples / tests etc.

[IvanUkhov]

Hello, sure if they are meaningful and what the intuition tells, that is, standard.

[saresend]

#35 - Yeah I'm not sure there are standard values for some of these in quite the same way there might be for a gaussian or uniform distribution, but I think even still there'd be value to have some default implementation for all random variables, rather than for only a subset.