fixes for recent buffer function changes + improvements to buffer-read-as-wavetable

[defaultxr]

This commit fixes some bugs I introduced/overlooked when changing &optional to &key in buffer-get* functions in a recent commit (sorry about that)...

Plus it includes improvements for buffer-read-as-wavetable:

• accept bufnum and server keyword arguments
• compute powers-of-two at compile time
• use buffer-to-list instead of buffer-get-to-list