Rework default style handling

There are a number of issues with the way the default style is handled now.  There is no way for a user to override the defaults with the compiled lib (`ol.style.Style.defaults` is not exported).  In addition, because the default style instance is created unconditionally, all of the style stuff (including expression parsing) is always included in all builds (even those that don't use a vector layer).

This can be improved by adding `ol.style.setDefault` and `ol.style.getDefault` functions (with the default created lazily).