Add function to define parameter from command line

This patch is based on one from "bruce <[email protected]>".
I've applied all but the elaboration code, which I rewrote to
properly work with the elaboration work queue. I also constrained
the implementation so that the parameter name must have exactly
two components: the root scope name and the parameter name. This
is necessary to keep the defparm processing sane. The comments
from bruce's original patch are as follows:
--
This patch would provide function to define parameter from command
line. This serves the same functionality as 'defparam' in Verilog
source code, but provide much more ease for using. Parameter
definition can be write in command file, with following syntax:

    +parameter+<scope>.<parameter>=<val>

*Do not apply any space between them*
The scope name should be full hierachical name with root name at
the begining. The following example would override test.T1 with
new value 2'b01:

    +parameter+test.T1=2'b01

'test' here is the root module name. The parameter value here
should be constant. Parameter definition can also be write in
the command line:

    iverilog -Ptest.T1=2'b01

This serves the same functionality with the previous example.
If we define the same parameter in command file and command line,
the one in command line would over-write all others.

Module.cc

@@ -24,6 +24,8 @@
 # include  "PWire.h"
 # include  <assert.h>
 
+list<Module::named_expr_t> Module::user_defparms;
+
 /* n is a permallocated string. */
 Module::Module(perm_string n)
 : PScope(n)