hand/lexer: Add support for lexing tokens starting with '!'.

uc/hand/lexer/lexer.go

@@ -148,6 +148,9 @@ func (l *lexer) next() (r rune) {
 // backup backs up one rune in the input. It can only be called once per call to
 // next.
 func (l *lexer) backup() {
+	if l.width == 0 {
+		panic("invalid call to backup; no matching call to next")
+	}
 	l.cur -= l.width
 	l.width = 0
 }

uc/hand/lexer/state.go

@@ -98,6 +98,8 @@ func lexToken(l *lexer) stateFn {
 		return lexAmpersand
 	case '\'':
 		return lexCharLit
+	case '!':
+		return lexExclaim
 	}
 
 	// Lex integer literal.