Access to arrays is broken for non-array accesses

[kikofernandez]

The problem is that the parser considers an ArrayAccess anything that matches x(y) = z. The typechecker goes along and considers that the AST node given by the parser is an array, and tries to typecheck it incorrectly, crashing the compiler.

This has been shown in #823 and also in this example:

active class Main
  def main(): unit
    val x = 3
    x(1) = 2
  end
end

Error thrown:

encorec: No match in record selector argTypes