Add support for SystemVerilog-style `define to Verilog frontend

This patch should support things like

  `define foo(a, b = 3, c)   a+b+c

  `foo(1, ,2)

which will evaluate to 1+3+2. It also spots mistakes like

  `foo(1)

(the 3rd argument doesn't have a default value, so a call site is
required to set it).

Most of the patch is a simple parser for the format in preproc.cc, but
I've also taken the opportunity to wrap up the "name -> definition"
map in a type, rather than use multiple std::map's.

Since this type needs to be visible to code that touches defines, I've
pulled it (and the frontend_verilog_preproc declaration) out into a
new file at frontends/verilog/preproc.h and included that where
necessary.

Finally, the patch adds a few tests in tests/various to check that we
are parsing everything correctly.

tests/various/sv_defines_mismatch.ys

@@ -0,0 +1,5 @@
+# Check that we spot mismatched brackets
+logger -expect error "Mismatched brackets in macro argument: \[ and }." 1
+read_verilog <<EOT
+`define foo(x=[1,2})
+EOT