Tabs mess up error reporting

[andymcn]

Tabs are counted as single characters by our lexer. This works fine for everything except printing out the indication of where in the line an error occurred. This will go wrong as the source will be printed with tabs in it, which will be expanded to multiple spaces by printf, but the location indication will be printed with space per source character, including one space per tab.

We could simply replace tabs with single spaces when printing out the source for errors.

Alternatively we could replace all tabs with spaces during lexing.

[andymcn]

Each tab in the source is now replaced by a single space by the lexer