Adding metered concurrency.
Adding better comments.
More comments for metered concurrency, and shorter backoff.
Turn statics into consts.
Replace all tabs by 4 spaces.
Merge branch 'master' of github.com:Hoverbear/rust-rosetta into static_to_const
Updating Cargo.lock for num now that it's fixed upstream.
remove not_tested comments and add trailing newlines

Here's the command.  Apparently sed adds trailing newlines too.

sed -i '' '\#^ *// *not_tested *$#d' src/*.rs

Closes #248
Merge pull request #270 from jschaf/remove-not-tested

remove not_tested comments and add trailing newlines
Merge pull request #264 from pythonesque/static_to_const

Turn statics into consts.
Remove unused import in closest-pair.rs.
Add K-d tree.
Merge pull request #269 from utkarshkukreti/replace-tabs-with-spaces

Replace all tabs by 4 spaces.
Merge pull request #271 from TimDumol/closest-pair-unused-import

Remove unused import in closest-pair.rs.
Merge pull request #260 from pythonesque/metered_concurrency

Adding metered concurrency.
Merge pull request #272 from TimDumol/kd-tree

Add K-d tree.