math/rand: export Source64, mainly for documentation value
math/rand: export Source64, mainly for documentation value

There is some code value too: types intending to implement
Source64 can write a conversion confirming that.

For #4254 and the Go 1.8 release notes.

Change-Id: I7fc350a84f3a963e4dab317ad228fa340dda5c66
Reviewed-on: https://go-review.googlesource.com/33456
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

Cofyc/go,cloudflare/go,suncycheng/go,cloudflare/go,akutz/go,ImJasonH/go,cloudflare/go,ImJasonH/go,golang/go,golang/go,nawawi/go,Cofyc/go,goodlang/good,polyverse-security/go,cloudflare/go,linux-on-ibm-z/go,cloudflare/go,Cofyc/go,cloudflare/go,ImJasonH/go,goodlang/good,codestation/go,josharian/go,linux-on-ibm-z/go,codestation/go,derekmarcotte/go,suncycheng/go,ImJasonH/go,polyverse-security/go,akutz/go,nawawi/go,polyverse-security/go,Cofyc/go,ImJasonH/go,polyverse-security/go,suncycheng/go,linux-on-ibm-z/go,ImJasonH/go,linux-on-ibm-z/go,derekmarcotte/go,nawawi/go,CAFxX/go,nawawi/go,polyverse-security/go,golang/go,codestation/go,ImJasonH/go,suncycheng/go,josharian/go,polyverse-security/go,codestation/go,codestation/go,josharian/go,derekmarcotte/go,akutz/go,golang/go,christopher-henderson/Go,polyverse-security/go,derekmarcotte/go,CAFxX/go,josharian/go,suncycheng/go,linux-on-ibm-z/go,cloudflare/go,goodlang/good,nawawi/go,CAFxX/go,golang/go,Cofyc/go,vsekhar/elastic-go,akutz/go,cloudflare/go,josharian/go,goodlang/good,nawawi/go,vsekhar/elastic-go,derekmarcotte/go,goodlang/good,vsekhar/elastic-go,suncycheng/go,goodlang/good,akutz/go,goodlang/good,vsekhar/elastic-go,suncycheng/go,vsekhar/elastic-go,Cofyc/go,derekmarcotte/go,codestation/go,codestation/go,CAFxX/go,ImJasonH/go,nawawi/go,suncycheng/go,golang/go,CAFxX/go,Cofyc/go,polyverse-security/go,akutz/go,josharian/go,derekmarcotte/go,nawawi/go,codestation/go,vsekhar/elastic-go,vsekhar/elastic-go,akutz/go,linux-on-ibm-z/go,CAFxX/go,CAFxX/go,vsekhar/elastic-go,josharian/go,Cofyc/go,linux-on-ibm-z/go,goodlang/good,CAFxX/go,derekmarcotte/go,josharian/go,golang/go,akutz/go,linux-on-ibm-z/go,golang/go