Fix precision problem.
Fix precision problem.

BigDecimal(BigInt(2).pow(n)) will be sure to preserve all
digits, whereas BigDecimal(2).pow(n) will lose digits for
large n.

This is something that worked in 2.10, but will fail in 2.11
so we might as well change it now.

codeaudit/spire,woparry/spire,adampingel/spire,AlecZorab/spire,JackSullivan/spire,kevinmeredith/spire,AlecZorab/spire,siddhartha-gadgil/spire,JackSullivan/spire,rklaehn/spire,non/spire,adampingel/spire,woparry/spire,tixxit/spire,kevinmeredith/spire,rklaehn/spire,codeaudit/spire,siddhartha-gadgil/spire,guersam/spire,non/spire,tixxit/spire,guersam/spire