[parser] Formalize KW_FOR as a keyword starting new lexical scope

Tests covering this changes:
- t/op/for-control-scope.t
- t/op/for-over-scope.t

* Motivation

Normalize where new lexical context starts so every variant
of for loop will start it at same position.

Such normalization will later allow reduction of code currently
duplicated.

* Change description

For statement already defined its own lexical scope in each its branch
though in different places for each branch (using <*> as a marker):

```
for (<*> $control = 0; ...)
for my <*> $cursor (...)
for my <*> ($cursor_a, $cursor_b) (...)
for $cursor (<*> ...)
for my \ <*> $cursor (...)
for \ my <*> $cursor (...)
for \ $cursor (<*> ... )
for (<*> ...)
```

This change formalizes every branch to start with:
```
for <*> ...
```

* Technical details

Lexical scope is started by non-terminal `remember` - empty with an action.

Bison's LALR(1) grammar evaluates action before looking which terminal token
follows, which means, that when `remember` occurs it must be either:
- preceded by unique sequence of terminal tokens (approach used before)
- be on same position for every possible branch (approach used by this change)

As far as `KW_FOR` (as statement) always starts new lexical scope,
it's possible to formalize it by starting it right after keyword,
when `for` statement is identified.

Author: Branislav Zahradník