Change float32/float64 formatting to use non-exponential form
for a slightly wider range, to more closely match ES6 JSON.stringify
and other JSON generators.

Most notably:

	1e20 now formats as 100000000000000000000 (previously 1e+20)
	1e-6 now formats as 0.000001 (previously 1e-06)
	1e-7 now formats as 1e-7 (previously 1e-07)

This also brings the int64 and float64 formatting in line with each other,
for all shared representable values. For example both int64(1234567)
and float64(1234567) now format as "1234567", where before the
float64 formatted as "1.234567e+06".

The only variation now compared to ES6 JSON.stringify is that
Go continues to encode negative zero as "-0", not "0", so that
the value continues to be preserved during JSON round trips.

Fixes #6384.
Fixes #14135.

Change-Id: Ib0e0e009cd9181d75edc0424a28fe776bcc5bbf8
Reviewed-on: https://go-review.googlesource.com/30371Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>