Changed the context syntax from "{context} exp" to "\=context exp".

I did that for two reasons:

1. I can now use braces to denote tuples, e.g. {x y z}, which seems like a
natural fit.

2. Definition of symbols with a context is conceptually similar to a lambda
form anyway, and the new syntax reflects that.  If you omit the symbol from a
lambda form, the context is used to define *all* symbols in the expression.