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

test/asm/multi-line-strings.err

@@ -1,2 +1,5 @@
-warning: multi-line-strings.asm(34): [-Wuser]
+ERROR: multi-line-strings.asm(23):
+    Unterminated string
+warning: multi-line-strings.asm(35): [-Wuser]
     check the line number
+error: Assembly aborted (1 errors)!