Add NaNConvertMode to EncOptions for v1.5

[x448]

NaNConvertMode will determine how to encode NaN and it will override ShortestFloatMode.

These two options are broken for sNaN values until a Go bug in reflect package is fixed golang/go#36400

• NaNConvertNone (much sad because this is for CTAP2 Canonical CBOR in FIDO2, WebAuthn)
• NaNConvertPreserveSignal (much sad because this is preferred in RFC 7049bis Draft 12 when protocols don't want to use 0x7e00 for every NaN value)

Complete list of options are:

• NaNConvertNone: never modify or convert NaN to other representations (float64 NaN stays float64, etc. even if it can use float16 without losing any bits)
• NaNConvertPreserveSignal: convert to smallest form that preserves value (signal status + payload) as described in RFC 7049bis Draft 12
• NaNConvert7e00: always encode to 0xf97e00 (CBOR float16 = 0x7e00)
• NaNConvertQuiet: always force quiet bit = 1 and shortest form that preserves NaN payload

EDIT: shortened option names and mention specific options affected by Go issue 36400 and make it clear this overrides ShortestFloatMode

[x448]

OK, so Keith Randall (randall77) came to the rescue by not only fixing the Go bugs but also offering workarounds that works today so we don't have to wait for Go 1.15 (it's too late for Go 1.14.)

His workaround for reflect issue doesn't require importing and using unsafe.

Compiler writers like Keith don't grow on trees and I feel lucky he gave us some of his very valuable time.

The last time I was similarly impressed by an open source issue resolution was years ago, and by coincidence it was also a Go issue but not mine. That one was impressive due to technical difficulty -- I regret not bookmarking it so my memory could be wrong, but I recall Austin Clements fixed a bug that eluded debugging by writing a proof and replacing the buggy code. It gave me nerd chills.

For different reasons that are remarkable (ones an older me appreciates), I'm going to remember how Keith handled the two floating-point issues reported by @fxamacker and me.