Regenerate.
Add tests for log2 and log10.
Fix bug in dd-%log2 where (dd-%log2 100w0) returned 6.16 instead of
6.64.

Tests were already added to tests/irrat.lisp.
Add log10 implementation for double-doubles.

Since log2 and log10 use basically the same natural log
implementation, factor that out the common part into its own routine.