[Agda] Allow certain whitespace characters in identifiers

[Peter Hancock]

For the evil among us on this list:

https://github.com/reinderien/mimic

Once, writing a paper with Conor, with minutes to go to the deadline,
we discovered that some mad interaction of lhs2tex, emacs and agda etc
had littered our source with bizarre and unfindable space-characters. 
It was I think the last paper I ever wrote. (It was rejected.)