To do fix: qs does not properly track active bindings in R6 (or environments)

[traversc]

Example:

library(qs)
library(R6)

XClass <- R6Class( "XClass", active = list(r=function() runif(1)) )
x <- XClass$new()

qsave(x, file="/tmp/test.z")
x2 <- qread("/tmp/test.z")

> x$r
[1] 0.8119341
> x2$r
function() runif(1)
<environment: 0x7f8dfc34f4f8>

[traversc]

Fixed on cran.