Rename "defs"?

[mgeier]

The submodule sfs.defs is supposed to mean "defaults".
This is quite accurate, since it contains default values.

However, if we would rename it to default, it would make more sense when reading it, e.g. sfs.default.c reads like "default c", not like "defaults c".

Apart from renaming, we could also turn it into an object of a custom class, like e.g. http://python-sounddevice.readthedocs.io/#sounddevice.default
This way, we could avoid accidentally assigning unsupported attributes.