Evaluate cost / benefit of "span" feature

[jimblandy]

The "span" feature is essential for producing usable diagnostics. As we improve Naga's diagnostics I suspect it's going to become harder to maintain "span" as an optional feature, as opposed to simply providing it by default.

We should measure the impact of the "span" feature on speed, memory consumption, and code size, and see what it would cost to simply enable it unconditionally.