`for each breakpoint` mixin

[ArmorDarks]

Merge constructions like

@if $ekzo-enable-responsive-widths {

    @each $breakpoint, $value in $ekzo-breakpoints {
        @include ekzo-breakpoint($breakpoint) {
            @include ekzo-widths('#{$breakpoint}-');
        }
    }

} // endif

into single each-breakpoint() mixin

[ArmorDarks]

Impossible to implement since so far Sass doesn't have way to call mixins:

Ref: sass/sass#626

[ArmorDarks]

Partially implemented, but via global variable hack and additional ekzo-temp() function.

It's still requires to have @content, since there is no way to call mixin with unknown name.