Just a cleanup for easier-to-read errors
Adding currying support for the receiver object in field lookup (colon, dotted, bracketed and colon-bracketed), object extension, object update, and object mutable-lookup.  This complicates desugaring of functions, to make sure that the function lookup (_.f) isn't prematurely desugared as a curried-field lookup...  Also fixing a bug in desugaring curried functions that left a s-dot AST node in the desugared output.
Merge branch 'master' of github.com:brownplt/pyret-lang