Both `alternative` and `monad` implement `applicative`. However,
their default implementations for `seq_right` and `seq_left` are
different. The `alternative` implementation uses the inefficient default
version for `seq_right` available at `applicative`:
```
(seq_right := λ α β a b, const α id <$> a <*> b)
```
instead of the more efficient
```
(seq_right := λ α β x y, x >>= λ _, y)
```
defined at `monad` using the `bind` operator.
This commit makes sure the `applicative` instances for `reader_t`,
`state_t`, `option` and `parsec_t` use the efficient version.
I found the problem when inspecting the generated code for:
```
def symbol (s : string) : parsec' unit :=
(str s *> whitespace) <?> ("'" ++ s ++ "'")
```
@Kha, I was having to use `(run ...).1` in many places
in my code. So, I changed the instance
```
instance (σ m out) [monad_run out m] : monad_run (λ α, σ → out (α × σ)) (state_t σ m) :=
⟨λ α x, run ∘ (λ σ, x.run σ)⟩
```
to
```
instance (σ : Type u) (m out : Type u → Type v) [functor m] [monad_run out m] : monad_run (λ α, σ → out α) (state_t σ m) :=
⟨λ α x, run ∘ (λ σ, prod.fst <$> (x.run σ))⟩
```
If we want to get the resultant state in an action `act`, we can just
use `run (act >> get) init_state`.
