Improve error messages

[amitp]

James Harr would like:

• Line & Character Position

• Parse Stack

• List of expected Tokens

• Parse string

• A function to generate something similar to this:

  Parse error near line 86, character 4:
    int x = 5;
    cahr y = 'c';
    ^^^^
  Found "cahr", expected [char, int, bool, array].