Loop statement support about veryl

[dalance]

I'll add Rust style for statement like below:

for i: u32 in 0..MAX {
}

This style can't specify the range step. ( In Rust, it can be (0..MAX).step_by(2) )
So the additional notation to specify step ( or other incrementation ) is needed.

for i: u32 in 0..MAX += 2 {
}

This is just idea.

[taichi-ishitani]

for i: u32 in 0..MAX += 2

This notation looks like below.

for i: u32 in 0..(MAX += 2) 

So I think it is better to introduce a separator symbol or keyword.

for i: u32 in 0..MAX step_by 2

[dalance]

The syntax of range will be EXPRESSION .. EXPRESSION,
and += will be statement. so (MAX += 2) is invalid.

On the other hand, the same symbol may be confusable.
The following notation can specify more operation.

0..MAX step += 2
0..MAX step -= 2
0..MAX step *= 2
0..MAX step /= 2

[taichi-ishitani]

0..MAX step += 2

This notation looks good for me.

[dalance]

Now for/if statement and for/if declaration ( a.k.a generate for/if ) are supported.