🚧 manual lexer

This commit removed the re2c lexer and replaced it by a manual version.
Its integration is not yet complete: number parsing does not respect
locales or overflows. Furthermore, parsing does not need to end with
EOF. Therefore, a lot of test cases fail. The idea is to push this
branch forward so we can conduct performance comparisons. So far, a
nice side effect are better diagnosis messages in case of parse errors.

errors.txt

@@ -0,0 +1,7 @@
+- test/test-class_parser
+    - 617 failed
+- test/test-regression
+    - 11 failed
+- test/test-testsuites
+    - 43 failed
+