Introduce a public constant for Rodio's default sample rate

[PetrGlad]

This should help to reduce number of sample rate conversions. Setting it to current de-facto default (44.1kHz).

There is still a question whether we should use cpal::SampleRate or plain integer. With either choice it is better to use the same type everywhere in the API. Currently some functions accept cpal::SampleRate while others use u32. cpal::SampleRate helps with type-checks but does not really provide useful services.

Also partially relieves #208. The rate converter has to be fixed still but with default sample rate less resampling is needed.

[PetrGlad]

A more advanced solution could allow processing steps to query current preferred sample rate and use that. With such API one could for example open output stream with 48kHz and all processing steps attached to it will adjust accordingly. Or make sample rate a parameter for every processing step that may be affected by it. Which cumbersome but could still help keeping 48kHz everywhere if one needs that. Alternatively it could be some global switch that can be read by all Rodio code, but I am not sure I like it since users may want more than one output stream with different sample rates.
But these are a lot bigger changes, and I do not have a clear idea how to implement those.