…ints and runes

Also switch float64 NaN encoding to use hexadecimal, and accept
hexadecimal encoding for all other integer types too. (That gives us
the flexibility to change the encodings in either direction in the
future without breaking earlier Go versions.)

Out-of-range runes encoded using "%q" were previously replaced with
the Unicode replacement charecter, losing their values.

Out-of-range ints and uints on 32-bit platforms were previously
rejected. Now they are wrapped instead: an “interesting” case with a
large int or uint found on a 64-bit platform likely remains
interesting on a 32-bit platform, even if the specific values differ.

To verify the above changes, I have made TestMarshalUnmarshal accept
(and check for) arbitrary differences between input and output, and
added tests cases that include values in valid but non-canonical
encodings.

I have also added round-trip fuzz tests in the opposite direction for
most of the types affected by this change, verifying that a marshaled
value unmarshals to the same bitwise value.

Updates #51258
Updates #51526
Fixes #51528

Change-Id: I7727a9d0582d81be0d954529545678a4374e88ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/390424
Trust: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
(cherry picked from commit 7419bb3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/390816
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>