Return validation errors as maps

Closes #286 
However it generates more sophisticated code then previous one:
```elixir
struct(user, age: present() and greater_than(18) when user.name != "jose")
```
```elixir
if(user.name() != "jose") do
  present(:age, Map.get(var, :age)) ++ greater_than(:age, Map.get(var, :age), 18)
else
  []
end
# vs
if(user.name() != "jose") do
  case({acc = case({acc = %{}, greater_than(:age, Map.get(var, :age), 18)}) do
    {_, []} ->
      acc
    {%{age: msgs}, [{_, msg}]} ->
      Map.put(acc, :age, [msg | msgs])
    {_, [{_, msg}]} ->
      Map.put(acc, :age, [msg])
  end, present(:age, Map.get(var, :age))}) do
    {_, []} ->
      acc
    {%{age: msgs}, [{_, msg}]} ->
      Map.put(acc, :age, [msg | msgs])
    {_, [{_, msg}]} ->
      Map.put(acc, :age, [msg])
  end
else
  %{}
end
```

One of the possible pros is that we can simplify current interface for predicates:
```elixir
# from
def image_attachments(attr, value, opts \\ []) do
  if Path.extname(value) in ~w(jpg gif png) do
    []
  else
    [{attr, opts[:message] || "is not an image attachment"}]
  end
end
# to
def image_attachments(attr, value, opts \\ []) do
  if Path.extname(value) in ~w(jpg gif png) do
    opts[:message] || "is not an image attachment"
  end
end
```
Regarding `Map.update/4`, I'm a little bit worried about performance, but I guess validations usually operate over small lists of arguments.