[Ur] Proposed location literals to track caller's location in error msgs.

[Adam Chlipala]

On 06/03/2015 09:26 AM, Gabriel Riba wrote:
> Here comes the mercurial patch with both the _LOC_ in urweb.lex and 
> the "assert" function in top.ur

Thanks, patch accepted.  I had to add some fixes to make it actually 
work in top.ur; I presume testing only happened with the code at some 
other location.