Copies each T from slices into a new slice that exactly holds all the elements.
pub fn concat(allocator: Allocator, comptime T: type, slices: []const []const T) Allocator.Error![]T
pub fn concat(allocator: Allocator, comptime T: type, slices: []const []const T) Allocator.Error![]T {
return concatMaybeSentinel(allocator, T, slices, null);
}