See nannou-org#752 for motivation behind adding a `nannou_core` crate.

Note that this commit does not yet replace the color, geom, math and
rand modules within the `nannou` crate. This will be performed in a
following commit.

This removes the color, math, rand and geom modules in favour of
depending on the `nannou_core` crate. The `nannou_core` modules are
re-exported within `nannou` in order to provide a near identical API to
the existing one.

The majority of changes here revolve around the change from `cgmath` to
`glam`. The most notable changes include:

- `geom::Vector2` -> `glam::Vec2`
- `cgmath::Matrix4<f32>` -> `glam::Mat4`
- `magnitude` -> `length`

This change also removes the generic `Scalar` type parameter from the
`Draw` API and related items in the hope of improving downstream compile
times.

Also removes the seemingly unused `geom::graph` module. Use-cases for
this module and its abstractions were mostly made reduntatnt upon
allowing for transformations to be applied to the `Draw` instance
itself.

This refactors the `Rect` and `Cuboid` methods to try to use `Point*`
and `Vec*` types where possible for `Rect<f32>` and `Rect<f64>` in order
to maintain an API closer to the original and avoid large sweeps amounts
of breaking changes.

Also exposes common `glam` types via the `nannou_core` `prelude` module.