WIP: fix slowness of randn introduced by 84b6aec6

Quoting @rfourquet:

@ViralBShah Great! I remember having tested inlined rand() at some point (before #8854 I think, and without other `@inlines` of this commit) but noticing degraded performances somewhere else (I think for the array version rand(n::Int)). What I find now is that this commit slows down randn[!] methods by up to 50%, do you confirm?

I just pushed a branch where randmtzig_randn uses a non-inlined version of rand which seem to restore previous perfs of randn, but this starts to make me feel uneasy with all those `@inline` tweaks. Should I open a PR? (In this branch, I also revert/modify part of this commit because I think rand_close_open is dSFMT specific so I prefer to keep rand(r::AbstractRNG) = rand(r, Float64) which I find more self-documenting, do you agree?).