Tweak `Span` encoding.
Tweak `Span` encoding.

Failing to fit `base` is more common than failing to fit `len`.

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