Cleanup decimal64const.go

[MichaelDao]

Let's clean this up.

1. Use consistent prefixes.
2. Suffix everything with 64.
3. Show the math behind the values.

Thus:

const (
	decDigits64  = 16
	expBits64    = 10
	expRange64   = 3 << (expBits - 2)
	symmMaxExp64 = expRange64 / 2
	symmMinExp64 = 1 - symmMaxExp64
	maxExp64     = symmMaxExp64 - (decDigits64 - 1)
	minExp64     = symmMinExp64 - (decDigits64 - 1)
)

We end up with symm(Max|Min)Exp64 = 384|-383 and (max|min)Exp64 = 369|-398.

The decimal128 group can then be written in exactly the same form, with the only changes being to the first two constants:

	decDigits128  = 34
	expBits128    = 14

It could even be extended to decimal32, in the event that we ever wanted to implement it:

	decDigits32  = 7
	expBits32    = 8

---

Technically, we could even derive decDigits(32|64|128):

	decDigits64  = (64 - expBits64) * 3 / 10

64 - expBits64 is the number of bits of precision available for the significand and 3/10 is close enough to log(2)/log(10) to convert bits to digits for the given values of expBits(32|64|128). But I think this a bit too abstruse for the reader.

https://play.golang.org/p/X-WNwWgkcht