Parentheses in macro args prevent commas from starting new arguments

This is similar to C's behavior, and convenient for passing
function calls as single values, like `MUL(3.0, 4.0)` or
`STRSUB("str", 2, 1)`.

Fixes #704

test/asm/macro-arg-parentheses.out

@@ -0,0 +1,11 @@
+$C0000
+a(b,c,d)
+((e,f),g)
+))h
+i,j
+(k
+l)
+(m:)n,o(p)q)
+(r,s)t
+"))u,v("
+("w,x","y,z")