Test that cargo readme output matches README.md

[joshlf]

We use the following command to auto-generate the contents of README.md from the crate-level doc comment in src/lib.rs:

cargo readme | sed 's/\[\(`[^`]*`\)]/\1/g'

The sed command removes code links like:

/// Here is a link to [`Vec<u8>`].

We should add a test that the output of that command matches the current contents of README.md, which will ensure PRs which edit that doc comment but do not update README.md can't be spuriously merged.

[joshlf]

Fixed in fa7e3e8.