math/big: rename (*Float).Format to (*Float).Text

math/big: rename (*Float).Format to (*Float).Text

This paves the way for a fmt-compatible (*Float).Format method.
A better name then Text is still desirable (suggestions welcome).

This is partly fixing issue #10938.

Change-Id: I59c20a8cee11f5dba059fe0f38b414fe75f2ab13
Reviewed-on: https://go-review.googlesource.com/10493
Reviewed-by: Alan Donovan <adonovan@google.com>