Base-effects depends strictly on 4.12

[hyphenrf]

Is there a reason why this is the case?

multicore-opam/packages/base-effects/base-effects.base/opam

Line 7 in 3fe46b9

"ocaml" {>= "4.12.0"}

Especially considering that it allows for ocaml-variants of 4.02.2 to 4.12?

multicore-opam/packages/base-effects/base-effects.base/opam

Lines 8 to 14 in 3fe46b9

	"ocaml-variants" {
	= "4.12.0+domains+effects" |
	= "4.10.0+multicore" |
	= "4.06.1+multicore" |
	= "4.04.2+multicore" |
	= "4.02.2+multicore"
	}