Fix grammar for chars

Character literals `'\'` and `'''` are allowed.

asm.l

@@ -71,8 +71,8 @@
 
 ","                         { return COMMA; }
 ";"(.*)\n                   { ; }
-\'\\?.\'                    { yylval->string = strdup(yytext); return CHAR; }
-\"(\\.|[^\"])*\"            { yylval->string = strdup(yytext); return STRING; }
+\'(\\.|[^\\\'\n])\'         { yylval->string = strdup(yytext); return CHAR; }
+\"(\\.|[^\\\"\n])*\"        { yylval->string = strdup(yytext); return STRING; }
 -?[01]+[bB]                 { yylval->string = strdup(yytext); return NUMBER; }
 -?[0-9][0-9A-Fa-f]*[hH]     { yylval->string = strdup(yytext); return NUMBER; }
 -?[0-9]+                    { yylval->string = strdup(yytext); return NUMBER; }

tests/parsing/fail/char_lf.asm

@@ -0,0 +1,2 @@
+push '
+'

tests/parsing/fail/char_slash_lf.asm

@@ -0,0 +1,2 @@
+push '\
+'

tests/parsing/regress/fail/char_slash_unterminated.asm

@@ -0,0 +1,2 @@
+; This was parsed as push '\''
+push '\'

tests/parsing/regress/fail/char_unescaped_quote.asm

@@ -0,0 +1,2 @@
+; This was parsed as push '\''
+push '''