genrams: when pre-loading a RAM with an init file, assert that the file size is not bigger than the size of the memory it is written to

genrams: when pre-loading a RAM with an init file, assert that the file size is not bigger than the size of the memory it is written to

genram: automatically pad with zeroes when reading from a mem init file smaller than the memory itself

New function expects textfile with binary representation of each word in a
separate line, e.g.:
10011000000000000000000000000000
11010000000000000000000000000000
11010000001000000000000000000000
01111000000000010000000000000000
00111000001000010000000000000000

This for example reduces WRPC SPEC reference design synthesis time from ~26
minutes to ~11 minutes.

On Altera, "" generates a null-range warning.
When the dpram is used heavily, these warnings add up to many.

Quartus will not process a 'file_open' call during synthesis,
so we can instead initialize the RAM with a vhdl constant.