[release-branch.go1.13] math/big: make Rat accessors safe for concurrent use
    
    Do not modify the underlying Rat denominator when calling
    one of the accessors Float32, Float64; verify that we don't
    modify the Rat denominator when calling Inv, Sign, IsInt, Num.
    
    For #36689.
    For #34919.
    For #33792.
    
    Change-Id: Ife6d1252373f493a597398ee51e7b5695b708df5
    Reviewed-on: https://go-review.googlesource.com/c/go/+/201205
    Reviewed-by: Ian Lance Taylor <iant@golang.org>
    Reviewed-on: https://go-review.googlesource.com/c/go/+/233321
    Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>