When resizing mesh and curves attribute storage, avoid initializing the
new memory for basic types. Also, avoid skipping "no free" layers; all
layers should be reallocated to the new size since they may be accessed.

The semantics introduced in 25237d2625078c6d1 are essential for this
change, because otherwise we don't have a way to construct non-trivial
types in the new memory.

In a basic test of the extrude node, I observed a performance
improvement of about 30%, from 55ms to 42ms.

The commit also includes utilities I found helpful for the change:
an attribute API function to retrieve a layer as a span without creating
it, and more functions for generic spans to mirror the typed versions.
I'll commit these parts separately though.