Add expression statements

example/helloworld.lang

@@ -1,7 +1,7 @@
 import lang.stdlib;
 
 (u8, u8) a(u8 a, u8 b) {
-	bool a = true;
+	bool c = true;
 	return a, b;
 }
 

parser.go

@@ -719,6 +719,20 @@ func (p *Parser) expectStatement() (*Statement, error) {
 		return stmt, nil
 	}
 
+	expr, err := p.tryExpression()
+	if err != nil {
+		return nil, err
+	}
+
+	if expr != nil {
+		_, err := p.expectSeparator(Separator_Semicolon)
+		if err != nil {
+			return nil, err
+		}
+
+		return &Statement{Type: Statement_Expression, Value: ExpressionStatement{Expression: *expr}}, nil
+	}
+
 	return nil, p.error("expected statement")
 }