Correct remarks about performance compared to Binary

This also improves the benchmark to be more sure about the issue.
Remarks about computational efficiency of type encodings

[skip ci]
Blank comment lines to hopefully fix Hackage's docs
Fix caching not working in encodeTyped

The "let" was inside the lambda, requiring the type information to be
recalculated on each invocation. Using ScopedTypeVariables, this can be
floated out.