Allowing global variables to be marked as top level clock inputs

[JulianKemmerer]

Related to allowing global variables to be top level inputs or outputs #123

Allow special clock enable and reset signals marked this way too maybe?

Otherwise, work around like in the graphics repo work - simply using a MAIN top level input port ex. pixel(uint1_t clock) that renders a pixel_clock input wire and marking it with the CLK_MHZ pragma as a clock doesnt work for all tools:

During internal runs of the tool all inputs have registers so timing paths are all sync. In this case the input clock is registered using itself as a clock and weird things happen...

At least apparently for some tools like Quartus, which, similar to #137, seem unable to use get_nets for the internal net name that isn't registered as the source of the clock for create_clock in the timing constraints. The internal clock net is also connected as an output port. Quartus can apply a clock to this output port with get_ports - but the clock constraint does not reach the interior of the design - internal registers,etc that are using the internal 'copy' of the clock wire are all still unconstrained/not reached by the output port create clock constraint.

Vivado however is able to define a proper internal clock net usingcreate_clock with get_nets on the intended top level clock signal (and seems to ignore preceding funny clock input register business).