Error tolerant parser

[tritao]

I'm playing with some changes for making the parser more error-tolerant, so it does not fail on first error (by keeping a list of emitted diagnostics during parse), adding an ND_ERROR node and overall improving robustness for invalid code.

Is this something you think could be interesting to add as a PR contribution?

For example:

struct foo {

}

enum bar {
    
}

union qux {
    
}

int main(void) {
    int a = 0;
    return 0;
}

=== Diagnostics ===
expected ';' after struct declaration
empty enum specifier
expected ';' after enum declaration
expected ';' after union declaration

[fuhsnn]

Hi, the frontend isn't complete nor stable, I still frequently find things necessary to change around for old bugs / new features, failing early keeps things more flexible. There are also loads of assumptions along the lines of "if control reaches here this pointer must be valid" that I imagine would be boilerplate-y to catch. So I consider this out-of-scope right now.