Add support for multidimensional arrays (HAST-255)

[Piedone]

Currently only single-dimensional arrays are supported, there is an explicit check for this too in ArrayCreateExpressionTransformer. However Vivado appears to support multi-dimensional arrays (at least for synthesis; further checks are necessary) like below, so we could transform into them:

type singleArray is array (0 to 5) of std_logic_vector(31 downto 0);
type multiArray is array (0 to 3) of singleArray;

type matrix is array (natural range <>, natural range <>, natural range <>) of std_logic_vector(31 downto 0);

Since the support for this can be dependent on tooling probably IDeviceDriver needs to be augmented too to indicate support for multi-dimensional arrays.

Since array.Length is handled now, GetLength() needs to be implemented in a similar manner (see the LENGTH attribute: https://www.csee.umbc.edu/portal/help/VHDL/attribute.html).

Also see: http://stackoverflow.com/questions/30651269/synthesizable-multidimensional-arrays-in-vhdl

Jira issue