Add support for immutable frame abstractions

[udoprog]

First step towards #3

This adds the following traits:

• UniformBuf which allows for getting and iterating over frames.
• Frame and FrameMut which allows for access samples within a frame.

The name UniformBuf currently stems from how frame access presupposes that each channel in the audio buffer has a uniform number of samples. That means it currently won't be implemented for types which have variable channels such as audio::buf::Dynamic. At the moment this is not being specifically leveraged and UniformBuf might be merged into Buf.

TODO

Channel access is still considered a bit too special in that its the "default" through Buf::iter and Buf::get and the like. This will be renamed but would constitute a large change. Or we keep it as is, but then we need to determine whether channel of frame access should be the default.