Avoid `unwrap_or_else` in str indexing
Avoid `unwrap_or_else` in str indexing

This provides a small reduction of generated LLVM IR, and leads to a
simpler assembly code.

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