Copies each T from slices into a new slice that exactly holds all the elements.
pub fn concatWithSentinel(allocator: Allocator, comptime T: type, slices: []const []const T, comptime s: T) Allocator.Error![:s]T
pub fn concatWithSentinel(allocator: Allocator, comptime T: type, slices: []const []const T, comptime s: T) Allocator.Error![:s]T {
const ret = try concatMaybeSentinel(allocator, T, slices, s);
return ret[0 .. ret.len - 1 :s];
}