math/big: implement GobDecode/Encode for big.Float.

[OneOfOne]

I'll submit a CL tomorrow if there's interest.

[bradfitz]

Int and Rat have those methods, so it makes sense that Float should too. Please send a CL.

/cc @griesemer

[OneOfOne]

Alright

[OneOfOne]

Submitted a CL @ https://go-review.googlesource.com/21755

[gopherbot]

CL https://golang.org/cl/21755 mentions this issue.