* Regenerate exercise READMEs to pick up tweaks to problem description

The upstream problem specifications have been updated with various
formatting changes, whitespace changes, and clarifications to the description.

This pulls in all the recent changes.

* Inline the exercise README insert

Now that we're operating with a track-wide template for generating exercise
READMEs we no longer need a separate markdown document to use for hints
and instructions that apply to all the exercises in a track. These instructions
can be defined directly in the template.

This reverts commit 43e792bf55f3e53ae696b8480ba8fe73bac5bba0.

You can still override the track-level default template by adding a .meta/readme.go.tmpl for the exercise in question.

This removes the stray newline at the end of the generated READMEs.

We are working towards making exercises stand-alone—that is to say:
no more generating READMEs on the fly.

This will give maintainers more control over each individual exercise
README.

The template uses the Go text/template package, and the default templates
generate the exact same READMEs as we have been generating on the fly.