[dev.fuzz] testing: convert seed corpus values where possible

The types provided in f.Fuzz will be viewed as the
canonical types for fuzzing. If the type is different
for a seed corpus entry, then the testing package
will attempt to convert it. If it can't convert it,
f.Fuzz will fail.

Currently, this allows converting types that may result
in precision loss or a semantically different value.
For example, an int(-1) can be converted to uint even
though the value could be math.MaxUint64. There is a
TODO to consider improving this in the future.

Updates golang/go#45593

Change-Id: I2e752119662f46b68445d42b1ffa46dd30e9faea
Reviewed-on: https://go-review.googlesource.com/c/go/+/325702


Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Katie Hockman <katie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>