Make Span::initial() a const fn
Make Span::initial() a const fn

In Arret nearly every intermediate data structure contains a `Span` so
our generated code can be mapped back to source location. However,
they're rarely used until we need to report a diagnostic or generate
debugging information.

This led to a common pattern in unit tests where we use a dummy span to
test intermediate transformations where we aren't starting from actual
Arret source. This used to be a `const` called `EMPTY_SPAN` which can be
used to build other `const` data structures used for testing.

In etaoins/arret#160 we needed to switch to an `empty_span()` function
because there is no longer a `const` constructor for `Span`.

We can't make `Span::new` `const` yet because const functions are not
powerful enough. However, `Span::initial` is perfect for this use case.

brendanzab/codespan