math/big: implement/rename accessors for precision and rounding mode

math/big: implement/rename accessors for precision and rounding mode

Also: remove NewFloat - not needed anymore. Work-around for places
where has been used so far:

NewFloat(x, prec, mode) === new(Float).SetMode(mode).SetPrec(prec).SetFloat64(x)

However, if mode == ToNearestEven, SetMode is not needed. SetPrec
is needed if the default precision (53 after SetFloat64) is not
adequate.

TBR adonovan

Change-Id: Ifda12c479ba157f2dea306c32b47c7afbf31e759
Reviewed-on: https://go-review.googlesource.com/4842
Reviewed-by: Robert Griesemer <gri@golang.org>