Scalar index access

[alari]

Fixes #329

To use non-literal indices, the new brackets syntax is introduced:

opt: []string

-- Take an element of an array, or 0 element
opt!

-- Take the sixth element
opt!5 

-- the same as opt!
opt[0]

-- the same as opt!5
opt[5]

-- get by index
func get(idx: i16) -> string:
   <- opt[idx]

-- Can use any proper value getter as an argument
get(opt!)

data Complex:
  field: []i16

func other(c: Complex) -> string
  x <- get(c.field!55)
  <- x