This Pull Request changes the following:
- Implement redeclaration errors in the parser
- Remove redeclaration errors from the compiler (this is a big step towards #1907)
- Fix some failing tests on the way
This requires a slight change in our public api. The Parser new requires a full `Context` instead of just the `Interner` for parsing new code. This is required, because if multiple scripts are parsed (e.g. every input in the REPL) global variables must be checked for redeclarations.