Handle string literals within macro arguments (#685)

Fixes #683 and #691

The lexer's raw mode for reading macro args already attempted
to handle semicolons inside string literals, versus outside ones
which start comments. This change reuses the same function for
reading string literals in normal and raw modes, also handling:

- Commas in strings versus between macro args
- Character escapes
- {Interpolations} and \1-\9 args inside vs. outside strings
- Multi-line string literals

Macro args now allow escaping '\', '"', and '\\'.

A consistent model for expanding macro args and interpolations,
within macro args, string literals, and normal context:

- "{S}" should always equal the contents of S
- "\1" should always act like quoting the value of \1

src/asm/rgbasm.5

@@ -235,7 +235,6 @@ There are a number of escape sequences you can use within a string:
 .It Sy String Ta Sy Meaning
 .It Ql \[rs]\[rs] Ta Produces a backslash
 .It Ql \[rs]" Ta Produces a double quote without terminating
-.It Ql \[rs], Ta Comma
 .It Ql \[rs]{ Ta Curly bracket left
 .It Ql \[rs]} Ta Curly bracket right
 .It Ql \[rs]n Ta Newline ($0A)
@@ -1088,6 +1087,10 @@ definition
 ENDM
 .Ed
 .El
+.Pp
+Macro arguments support all the escape sequences of strings, as well as
+.Ql \[rs],
+to escape commas, since those otherwise separate arguments.
 .Ss Exporting and importing symbols
 Importing and exporting of symbols is a feature that is very useful when your project spans many source files and, for example, you need to jump to a routine defined in another file.
 .Pp
@@ -1462,16 +1465,13 @@ PrintMacro: MACRO
 ENDM
 
     PrintMacro STRCAT("Hello "\[rs], \[rs]
-                      "world\[rs]\[rs]n")
+                      "world\[rs]n")
 .Ed
 .Pp
 The comma needs to be escaped to avoid it being treated as separating the macro's arguments.
-The backslash
-.Sq \[rs]
-.Pq from Sq \[rs]n
-also needs to be escaped because of the way
-.Nm
-processes macro arguments.
+The backslash in
+.Ql \[rs]n
+does not need to be escaped because string literals also work as usual inside macro arguments.
 .Pp
 In reality, up to 256 arguments can be passed to a macro, but you can only use the first 9 like this.
 If you want to use the rest, you need to use the