Cleanup: Inline `struct_span_fatal()`, which is only called once, and remove an outdated FIXME.
Cleanup: Inline `struct_span_fatal()`, which is only called once, and remove an outdated FIXME.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust