Allow NUL characters in strings (#1405)

test/asm/null-character.asm

@@ -0,0 +1,12 @@
+MACRO echo
+	print "\#"
+ENDM
+	; '\0' can be printed like any other character
+	print "hello\0world\0"
+	echo left\0right\0
+
+SECTION "test", ROM0
+	; '\0' can be included in ROM like any other character
+	db "foo\0bar", 0
+	charmap "a\0b", $42
+	db "a\0b\0"

test/asm/null-character.err

@@ -0,0 +1,2 @@
+warning: null-character.asm(12): [-Wunmapped-char]
+    Unmapped character '\0'

test/asm/null-in-macro.err

@@ -1,3 +1,3 @@
 error: null-in-macro.asm(4) -> null-in-macro.asm::foo(2):
-    Unknown character 0x00
+    Unknown character '\0'
 error: Assembly aborted (1 error)!

test/asm/null-outside-string.asm

@@ -0,0 +1,7 @@
+SECTION "test", ROM0
+	; '\0' is not special here; it's lexed as a line continuation...
+	DEF foo\0bar EQU 42
+	db foo\0bar
+	; ...just like any other non-whitespace character
+	DEF spam\Xeggs EQU 69
+	db spam\Xeggs

test/asm/null-outside-string.err

@@ -0,0 +1,17 @@
+error: null-outside-string.asm(3):
+    Begun line continuation, but encountered character '0'
+error: null-outside-string.asm(3):
+    syntax error, unexpected number
+error: null-outside-string.asm(4):
+    Begun line continuation, but encountered character '0'
+error: null-outside-string.asm(4):
+    syntax error, unexpected number
+error: null-outside-string.asm(6):
+    Begun line continuation, but encountered character 'X'
+error: null-outside-string.asm(6):
+    syntax error, unexpected identifier
+error: null-outside-string.asm(7):
+    Begun line continuation, but encountered character 'X'
+error: null-outside-string.asm(7):
+    syntax error, unexpected identifier
+error: Assembly aborted (8 errors)!

test/asm/symbol-invalid-macro-arg.asm

@@ -1,2 +1,2 @@
-def x\0 = 10
+def x\<0> = 10
 println x

test/asm/symbol-invalid-macro-arg.err

@@ -1,3 +1,3 @@
 error: symbol-invalid-macro-arg.asm(1):
-    Invalid macro argument '\0'
+    Invalid bracketed macro argument '\<0>'
 error: Assembly aborted (1 error)!